Hi @Community Alums 

The answer to your problem lies in the logic of the OOB Business Rule "Ensure min active admin remains" on sys_user table which refers to a Script Include "ScopedAdministration". As per this Script Include logic, at least 1 application administrator should remain active. Refer below screenshot  :

I suspect your user is the only application administrator for some custom application that's why you are not able to mark it inactive. You can go through your Company Applications and see if the user is mapped somewhere or go through the user roles and see if any custom application admin role is mapped to the user.

Thanks and Regards

Amit Verma