Terraform - How to get App Service object id for azurerm key vault access policy?
When you read the description for azurerm_key_vault_access_policy property object_id, then you should know it could mean the web app principal Id.
And the azurerm_app_service.myApp.id that you put is not the principal Id, it's the app service resource Id. You should put the azurerm_app_service.myApp.identity.principal_id that associated with your web app. Take a look at the Attributes of the App Service Resource. Hope this will help you.
However, something not mentionned in the documentation is the need to specify an identity block in your app_service declaration.
identity { type = "SystemAssigned" }If you don't specify it, you might get an empty list as identity attribute.