There are two kinds of tasks:
Manual tasks are completed manually by task assignees.
Automated tasks complete automatically.
Basic Tasks are manual. Task assignees must complete them explicitly.
List tasks are automated. A list task completes automatically when all of its enabled subtasks are either completed or skipped. If a client attempts to directly complete a List Task, the request will fail.
Possible causes and suggested actions
A client can only complete a List task by completing and/or skipping all of its subtasks.
If it is necessary to be able to complete a List task without completing some of its subtasks, then those subtasks should be marked as optional so that they can be skipped.