custom feild in task for project management

i have been working on development of project managemnent module for designing team.
i have to add customize feild name Discipline in task wich has to go through a workflow.

on submitting thetimesheet i keep on getting getting error msg :
Row #3: Discipline cannot be “MEP”. It should be one of “Design Task”

The error occurs because ‘MEP’ is not listed as a valid option for the Discipline field.

Go to Customize Form - Timesheet
Find the Discipline field
If it’s a Select field add MEP in the Options list
If it’s a Link field make sure your linked DocType has MEP as a record
Also check the linked Design Task (DTASK-202…) and ensure MEP is an allowed discipline there

Save and retry submitting the timesheet.