This is working as designed. The task cannot be marked completed if there are still any ETCs remaining. Change the ETC 0 then the task status should be able to be changed to Completed.
If however, all the assignments show as 0 and the summary task shows as 0 check to see if there are any orphan assignments.
To do this run query:
select * from prassignment where prtaskid = <enter InternalID of the task>
If you find an assignment where either PRPENDACTSUM and PRPENDESTSUM > 0 that is the assignment causing the issue.