SUMMARY :-
Users cannot modify or Edit a timesheet, if that contains any UDF value that is presently disabled.

RESOLUTION :-
Enable that UDF value and then make the necessary changes to the timesheet. You should now be able to submit it.
(If required, disable to UDF value once you have made changes and submitted the timesheet).

Note :
To enable/disable UDF value, follow the steps given below :-
   – Click on User Defined Fields in the left menu under Administration
   – Click on the timesheet tab
   – Click on the UDF that you want to modify
   – Select the value that is disabled and click on the 'Enable' button, or select a value that you want to disable and click on 'Disable' button.