Terraform output error: Error: Variables not allowed

Question

I am really not sure why I am seeing this error. I know it's something simple and right in my face . My issue is how I am passing on the subnet_id in the subnet_mapping block. This is the code below:

main.tf

resource "aws_lb" "lb" {
  name                             = var.name
  internal                         = var.internal
  load_balancer_type               = var.lb_type
  enable_cross_zone_load_balancing = var.enable_cross_zone_load_balancing

  subnet_mapping {
    allocation_id                  = aws_eip.lb.id
    subnet_id                      = var.subnet_id[1]
  }
}

resource "aws_eip" "lb" {
  vpc                              = true
}

variables.tf

variable "name" {
  type        = string
}

variable "internal" {
  type        = bool
  default     = false
}

variable "lb_type" {
  type        = string
  default     = "network"
}

variable "enable_cross_zone_load_balancing" {
  type        = bool
  default     = true
}

variable "vpc" {
  type        = bool
  default     = true
}

variable "vpc_id" {
  type        = string
}

variable "subnet_id" {
  type        = list(string)
  default     = []
}

terragrunt.hcl

include {
  path = find_in_parent_folders()
}

dependency "test" {
  config_path = "../../../folder/test"
  mock_outputs = {
    vpc_id            = "vpc-12345"
    public_subnet_ids = ["subnet-1", "subnet-2"]
  }
}

# var to pass in to use the module specified in the terragrunt configuration above
inputs = {
  vpc_id    = dependency.test.outputs.vpc_id
  subnet_id = dependency.test.outputs.public_subnet_ids[1]
  xxx...

Terragrunt error

Error: Variables not allowed

 on <value for var.subnet_id> line 1:
 (source code not available)

Variables may not be used here.

I would appreciate some feedback. It has been a pain for the past few hours.

Answer 1

This error message is quite misleading, as evidenced by several people raising the issue on GitHub. In your case, the problem is that you have a variable named subnet_id and you also use the same identifier in the inputs block (in subnet_id = dependency.test.outputs.public_subnet_ids[1]). Renaming one of them should solve the issue.