Scenario:
While trying to hide the client column, we do not see Display Clients on timesheet under Settings (User Preferences).

Cause:
The Display Clients on Timesheet will not be visible if Projects are created using Bucket Method. As Bucket Method method projects have multiple clients assigned to it, selecting the client becomes mandatory. Hence option to hide the client column will be not be available.
 

Related Items:

How to Hide/Display clients on the timesheet?