Contractors do not have permission to move an active case to the green Completed status through the Work Order Portal. Cases can only be marked as completed manually by the Building Manager.
However, you can create a custom status such as Contractor Completed, which contractors can select through their Work Order Portal. When selected, this status will appear on the Cases home screen, allowing the Building Manager to easily identify cases that contractors have indicated as completed.
This also gives the Building Manager the opportunity to review any updates or edits made by the contractor before manually moving the case to the final Completed status.
To create a contractor status:
Cases > Settings 
> Add/Edit Case Status > toggle Contractor on > Save
For more information on adding and editing Case Statuses, click here