A data resource "aws_subnets" "selected" has not been declared in the root │ module

I am using terragrunt. I have following declared in data.tf:

data "aws_vpcs" "this" {
  tags = {
    terraform    = "true"
    project_name = var.project_name
  }
}

data "aws_vpc" "this" {
  id = element(tolist(data.aws_vpcs.this.ids), 0)
}

data "aws_subnets" "selected" {
  filter {
    name   = "vpc-id"
    values = [data.aws_vpc.this.id]
  }
  tags = {
    environment  = var.environment
    project_name = var.project_name
    region       = var.region
    service      = "database"
  }
}

I want to use it in terraform-aws-modules/rds/aws module, as following:

  subnet_ids              = data.aws_subnets.selected.ids

But I get an error:

Error: Reference to undeclared resource
│ 
│   on rds.tf line 40, in module "rds_postgres_primary":
│   40:   subnet_ids              = data.aws_subnets.selected.ids
│ 
│ A data resource "aws_subnets" "selected" has not been declared in the root
│ module.

What am I doing wrong here? It used to work...

If I right understood you put this data.tf next to the terragrunt.hcl where you are passing information to the module rds_postgres_primary.

Please try instead declare in terragrunt.hcl:

locals {
  subnet_ids = data.aws_subnets.selected.ids
}

inputs = {
  subnet_ids              = local.subnet_ids
}