A client is forbidden to complete a task that is already DONE (i.e. completed or skipped).
Possible Causes and Suggested Actions
If a task is already completed (status = DONE, result = COMPLETED), then a client program does not need to complete it.
If a task is currently skipped (status = DONE, result = SKIPPED), then the client should reopen the task and then complete it.
For more about WAITING, IN_PROGRESS and DONE, see Task Life Cycle,
For more about Task Upload operations, see Task Upload.