unable to dynamically set a value in react-hook-form

Question

I am using react-hook-form to build a form. there is a section of the form where the user can enter item name, item price, and the quantity of items. i also want an input where the user cannot interact with, but rather calculates the total amount of the item based on the price and quantity the user entererd. but it seems i have been doing only the wrong things.

import React from "react";
import { useFieldArray, useForm } from "react-hook-form";
import { DevTool } from "@hookform/devtools";
const FormInput = () => {
  const {
    register,
    control,
    handleSubmit,
    formState,
    getValues,
    setValue,
    watch,
  } = useForm({
    defaultValues: {
      clientName: "Jensen Huang",
      clientEmail: "[email protected]",
      status: "paid",
      createdAt: new Date(),
      paymentDue: new Date(),
      description: "Re-branding",
      senderAddress: {
        street: "",
        city: "London",
        postCode: "E1 3EZ",
        country: "United Kingdom",
      },
      clientAddress: {
        street: "106 Kendell Street",
        city: "Sharrington",
        postCode: "NR24 5WQ",
        country: "United Kingdom",
      },
      items: [
        {
          name: "",
          quantity: "",
          price: "",
          // totalAmount: 0,
        },
      ],
    },
  });
  const onSubmit = (data) => {
    console.log(data);
  };
  const { errors, isValid, isDirty } = formState;
  const { append, remove, fields, insert } = useFieldArray({
    name: "items",
    control,
  });
  // Function to update total amount when quantity or price changes
  const calculateTotalAmount = (index) => {
    const quantity = getValues(`items.${index}.quantity`);
    const price = getValues(`items.${index}.price`);
    if (quantity && price) {
      const totalAmount = parseFloat(quantity) * parseFloat(price);
      setValue(`items.${index}.totalAmount`, totalAmount.toFixed(2));
    } else {
      setValue(`items.${index}.totalAmount`, "");
    }
  };

  return (
    <>
      <form onSubmit={handleSubmit(onSubmit)} noValidate>
        <div className="form-control">
          <h4>Bill From</h4>
          <div className="invoice-input">
            <label htmlFor="senderStreet">
              <span>Sender Address</span>
              <span className="error">
                {errors?.senderAddress?.street?.message}
              </span>
            </label>
            <input
              type="text"
              id="senderStreet"
              {...register("senderAddress.street", {
                required: {
                  value: true,
                  message: "can't be empty",
                },
              })}
            />
          </div>
          <div className="form-3-col">
            <div className="invoice-input">
              <label htmlFor="senderCity">
                <span>Sender City</span>
                <span className="error">
                  {errors?.senderAddress?.city?.message}
                </span>
              </label>
              <input
                type="text"
                id="senderCity"
                {...register("senderAddress.city", {
                  required: {
                    value: true,
                    message: "can't be empty",
                  },
                })}
              />
            </div>
            <div className="invoice-input">
              <label htmlFor="senderPostcode">
                <span>Post code</span>
                <span className="error">
                  {errors?.senderAddress?.postCode?.message}
                </span>
              </label>
              <input
                type="text"
                id="senderPostcode"
                {...register("senderAddress.postCode", {
                  // valueAsNumber: true,
                  required: {
                    value: true,
                    message: "can't be empty",
                  },

                  pattern: {
                    value: /^\d{1,5}$/,
                    message: "Please enter a number",
                  },
                  maxLength: {
                    value: 5,
                    message: "Postal code must be 5 characters or less",
                  },
                })}
              />
            </div>
            <div className="invoice-input">
              <label htmlFor="senderCountry">
                <span>Country</span>
                <span className="error">
                  {errors?.senderAddress?.country?.message}
                </span>
              </label>
              <input
                type="text"
                id="senderCountry"
                {...register("senderAddress.country", {
                  required: {
                    value: true,
                    message: "can't be empty",
                  },
                })}
              />
            </div>
          </div>
        </div>
        <div className="form-control">
          <h4>Bill to</h4>
          <div className="invoice-input">
            <label htmlFor="clientStreet">
              <span>Address</span>
              <span className="error">
                {errors?.clientAddress?.street?.message}
              </span>
            </label>
            <input
              type="text"
              id="clientStreet"
              {...register("clientAddress.street", {
                required: {
                  value: true,
                  message: "can't be empty",
                },
              })}
            />
          </div>
          <div className="form-3-col">
            <div className="invoice-input">
              <label htmlFor="clientCity">
                <span>city</span>
                <span className="error">
                  {errors?.clientAddress?.city?.message}
                </span>
              </label>
              <input
                type="text"
                id="clientCity"
                {...register("clientAddress.city", {
                  required: {
                    value: true,
                    message: "can't be empty",
                  },
                })}
              />
            </div>
            <div className="invoice-input">
              <label htmlFor="clientPostCode">
                <span>Post code</span>
                <span className="error">
                  {errors?.clientAddress?.postCode?.message}
                </span>
              </label>
              <input
                type="text"
                id="clientPostCode"
                {...register("clientAddress.postCode", {
                  required: {
                    value: true,
                    message: "can't be empty",
                  },

                  pattern: {
                    value: /^\d{1,5}$/,
                    message: "Please enter a number",
                  },
                  maxLength: {
                    value: 5,
                    message: "Postal code must be 5 characters or less",
                  },
                })}
              />
            </div>
            <div className="invoice-input">
              <label htmlFor="clientCountry">
                <span>country</span>
                <span className="error">
                  {errors?.clientAddress?.country?.message}
                </span>
              </label>
              <input
                type="text"
                id="clientCountry"
                {...register("clientAddress.country", {
                  required: {
                    value: true,
                    message: "can't be empty",
                  },
                })}
              />
            </div>
          </div>
        </div>
        <div className="form-control">
          <h4>Items</h4>
          {fields.map((field, index) => (
            <div key={field.id} className="form-4-col">
              <div className="invoice-input">
                <label htmlFor={`items.${index}.name`}>
                  <span>Item Name</span>
                </label>
                <input
                  type="text"
                  id={`items.${index}.name`}
                  {...register(`items.${index}.name`)}
                />
              </div>
              <div className="invoice-input">
                <label htmlFor={`items.${index}.quantity`}>
                  <span>Quantity</span>
                </label>
                <input
                  type="number"
                  id={`items.${index}.quantity`}
                  {...register(`items.${index}.quantity`)}
                  onChange={() => calculateTotalAmount(index)}
                />
              </div>
              <div className="invoice-input">
                <label htmlFor={`items.${index}.price`}>
                  <span>Price</span>
                </label>
                <input
                  type="text"
                  id={`items.${index}.price`}
                  {...register(`items.${index}.price`)}
                  onChange={() => calculateTotalAmount(index)}
                />
              </div>
              <div className="invoice-input">
                <label htmlFor={`items.${index}.totalAmount`}>
                  <span>Total Amount</span>
                </label>
                <input
                  type="text"
                  id={`items.${index}.totalAmount`}
                  {...register(`items.${index}.totalAmount`)}
                  readOnly
                />
              </div>

              <svg
                className="remove-item"
                onClick={() => remove(index)}
                width="13"
                height="16"
                xmlns="http://www.w3.org/2000/svg"
              >
                <path
                  d="M11.583 3.556v10.666c0 .982-.795 1.778-1.777 1.778H2.694a1.777 1.777 0 01-1.777-1.778V3.556h10.666zM8.473 0l.888.889h3.111v1.778H.028V.889h3.11L4.029 0h4.444z"
                  fill="#888EB0"
                  fill-rule="nonzero"
                />
              </svg>
            </div>
          ))}
          <div>
            <button
              className="btn"
              type="button"
              onClick={() =>
                insert(0, {
                  name: "",
                  quantity: "",
                  price: "",
                  totalAmount: "",
                })
              }
            >
              Add Item
            </button>
          </div>
        </div>

        <div className="btn-container">
          <div className="btn-discard">
            <button type="button" className="btn btn-discard">
              Discard
            </button>
          </div>
          <div className="btn-others">
            <button type="button" className="btn btn-draft">
              Save as Draft
            </button>
            <button type="submit" className="btn btn-send">
              Save & send
            </button>
          </div>
        </div>
      </form>
      <DevTool control={control} />
    </>
  );
};

export default FormInput;

i've tried multiple resources online but i'm only getting more confused!