How to return a data from Dataverse and assign to a variable inside Set Function in PowerApps

Question

I want to get data from the Dataverse table and then assign it to a variable. Below is my Set function.

In this line of code, I am trying to fetch the 1 index of my Table and get its value, and at the same time, I want to assign it to a SubHeaderValue variable.

Note: the state is a choice of multiple answers.

Set(SubHeaderValue, Index('myTableName', 1).state);

But I am getting the following error:

The formula has side effects.