Terraform - Unable to get subnet id

I'm trying to create RDS instance via terraform and I'm having trouble getting subnet id from AWS to create aws_db_subnet_group.

I've tried many methods such as:

data.aws_subnet.test_subnet.id

aws_subnet.test_subnet.id

but everytime I'm getting error: "A managed resource "aws_subnet" "test_subnet" has not been declared in the root module."

In the root location I've defined output variable:

  output "output" {
   value = {
    vpc_id              = module.vpc.vpc_data.vpc_id
    test_subnet_id      = module.vpc.vpc_data.test_subnet.*.id
  }
}

and I've also tried to use variable var.test_subnet_id but I was getting similar error:

An input variable with the name "test_subnet_id" has not been declared. This variable can be declared with a variable "test_subnet_id" {} block.

Does anyone knows what I'm doing wrong?

How you created VPC and subnets.

If you created manually you can get all IDs using following:

data "aws_subnet_ids" "subnet-ids" {
  vpc_id = {VPC-ID}
}

If you created by TF like:

resource "aws_subnet" "subnet1" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.0.1.0/24"
}

resource "aws_subnet" "subnet2" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.0.2.0/24"
}

Subnet IDs:

aws_subnet.subnet2.id
aws_subnet.subnet2.id

To create aws_db_subnet_group you can use:

resource "aws_db_subnet_group" "default" {
  name       = "main"
  subnet_ids = data.aws_subnet_ids.subnet-ids
}

or

resource "aws_db_subnet_group" "default" {
  name       = "main"
  subnet_ids = [ aws_subnet.subnet2.id, aws_subnet.subnet2.id]
}