When you create a new gradable LTI content and you mark it as a gradable item, it creates a new Gradebook column. This gradebook column is not accessible from the LTI Advantage tool provider using the Assignments and Grades Services, because it's been created by Learn, not the tool, which is sandboxed.
We can access this column using the REST API, but when we try to add a new attempt the response says that you can't do it...
"message":"Attempt operations not allowed for content of type resource/x-bb-bltiplacement-elea-tool",
It doesn't makes any sense me, are we misunderstanding something?