Error: Cannot index a set value

$ terraform -version
Terraform v1.2.9
on linux_amd64
# Locals Block for custom data
locals {
  webvm_custom_data = <<CUSTOM_DATA
#sudo yum update -y
sudo yum install -y httpd
sudo systemctl enable httpd
sudo systemctl start httpd  
sudo systemctl stop firewalld
sudo systemctl disable firewalld
sudo chmod -R 777 /var/www/html 
sudo echo "Welcome to stacksimplify - WebVM App1 - VM Hostname: $(hostname)" > /var/www/html/index.html
sudo mkdir /var/www/html/app1
sudo echo "Welcome to stacksimplify - WebVM App1 - VM Hostname: $(hostname)" > /var/www/html/app1/hostname.html
sudo echo "Welcome to stacksimplify - WebVM App1 - App Status Page" > /var/www/html/app1/status.html
sudo echo '<!DOCTYPE html> <html> <body style="background-color:rgb(250, 210, 210);"> <h1>Welcome to Stack Simplify - WebVM APP-1 </h1> <p>Terraform Demo</p> <p>Application Version: V1</p> </body></html>' | sudo tee /var/www/html/app1/index.html
sudo curl -H "Metadata:true" --noproxy "*" "" -o /var/www/html/app1/metadata.html

# Resource: Azure Linux Virtual Machine Scale Set - App1
resource "azurerm_linux_virtual_machine_scale_set" "web_vmss" {
  name = "${local.resource_name_prefix}-web-vmss"
  #computer_name_prefix = "vmss-app1" # if name argument is not valid one for VMs, we can use this for VM Names
  resource_group_name =
  location            = azurerm_resource_group.rg.location
  sku                 = "Standard_DS1_v2"
  instances           = 2
  admin_username      = "azureuser"

  admin_ssh_key {
    username   = "azureuser"
    public_key = file("${path.module}/ssh-keys/")

  source_image_reference {
    publisher = "RedHat"
    offer     = "RHEL"
    sku       = "83-gen2"
    version   = "latest"

  os_disk {
    storage_account_type = "Standard_LRS"
    caching              = "ReadWrite"

  upgrade_mode = "Automatic"

  network_interface {
    name                      = "web-vmss-nic"
    primary                   = true
    network_security_group_id =
    ip_configuration {
      name                                         = "internal"
      primary                                      = true
      subnet_id                                    =
      application_gateway_backend_address_pool_ids = [azurerm_application_gateway.web_ag.backend_address_pool[0].id]
  custom_data = base64encode(local.webvm_custom_data)
$ terraform validate
│ Error: Cannot index a set value
│   on line 59, in resource "azurerm_linux_virtual_machine_scale_set" "web_vmss":
│   59:       application_gateway_backend_address_pool_ids = [azurerm_application_gateway.web_ag.backend_address_pool[0].id]
│ Block type "backend_address_pool" is represented by a set of objects, and set elements do not have addressable keys. To find elements matching specific
│ criteria, use a "for" expression with an "if" clause.

Hi @efcunha,

The error you have here is trying to explain that azurerm_application_gateway.web_ag.backend_address_pool is a set, which means you cannot index into it because it is an unordered data structure. You can use a for expression to filter the values into an order collection if you have some criteria with which to select the propoer id.

If you only ever need a single value from a set of 1, you can use the one function to select a single set element.