You are unable to set the Task status as Completed and getting the error PRJ-07235: Invalid status for task having non-zero ETC, but the ETC for the task is zero.
The ETC may not be exactly zero.
One (or more) assignments on that task may still have ETC, that when rounded, is displayed as 0 (but this is not the same as exactly 0).
For any of those assignments, if any has a status other than Completed, manually update the value to 0 and save (or, update to other than 0, save, then update the value to 0 and save).