how to set deployment_mode when provisioning aws_mq_broker through terraform?

Question

I am trying to provision Amazon MQ broker through terraform. I have written code for multi AZ deployment with deployment type is ACTIVE_STANDBY_MULTI_AZ. Now I want to provision the MQ broker in Test environment with SINGLE_INSTANCE deployment type. hence I parameterized the deployment_mode field and passing the values in variables.

this is my variables list:

variable "enviroment" {
  default = "test"
}
variable "mq_multiAZ" {
  default = "SINGLE_INSTANCE"
}

The below code is absolutely working fine when I replaced the variable (mq_multiAZ) value to "ACTIVE_STANDBY_MULTI_AZ". however, it is not working with variable value "SINGLE_INSTANCE". also note- We require 2 subnets for "ACTIVE_STANDBY_MULTI_AZ" deployments, we can't mention single subnet to work "SINGLE_INSTANCE" deployment.

mq_broker.tf:

  resource "aws_mq_broker" "mymq_broker" {
  broker_name = "${var.enviroment}-broker"
  engine_type          = "ActiveMQ"
  engine_version       = "5.15.9"
  host_instance_type   = "mq.t2.micro"
  deployment_mode      = "${var.mq_multiAZ}"
  publicly_accessible  = false
  apply_immediately    = false
  security_groups      = [aws_security_group.amazon_mq.id]
  subnet_ids = [
    data.aws_subnet.AppSubnetA.id,
    data.aws_subnet.AppSubnetB.id,
  ]

  user {
    username = "${var.mq_master_user}"
    password = "${var.mq_master_pwd}"
    console_access = true
  }

  logs {
    general = true
  }

  maintenance_window_start_time {
    day_of_week = "SUNDAY"
    time_of_day = "02:00"
    time_zone   = "UTC"
  }

  tags = {
    Environment = "${var.enviroment}"
    Name        = "${var.enviroment}-broker"
  }
}

The error I am getting for "SINGLE_INSTANCE" deployment:

 Error: BadRequestException: Specify a single subnet in [SINGLE_INSTANCE] deployment mode.
 {
   RespMetadata: {
     StatusCode: 400,
     RequestID: "716aafdf-578a-4eb7-bfe4-f0f08998b6db"
   },
   ErrorAttribute: "subnetIds",
   Message_: "Specify a single subnet in [SINGLE_INSTANCE] deployment mode."
 }

   with aws_mq_broker.empays_broker,
   on amazonMQ.tf line 1, in resource "aws_mq_broker" "empays_broker":
    1: resource "aws_mq_broker" "empays_broker" {