Error: Cannot index a set value

[efcunha]

on c7-03-web-linux-vmss-resource.tf line 60, in resource "azurerm_linux_virtual_machine_scale_set" "web_vmss":
│ 60: 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.

[israrsache86]

same error showing