Summary:
All the users of Web TimeSheet in my organization submit expenses from time to time. They submit their expense sheets without selecting the payment method. This forces the expense approvers to reject the expense sheets.

Resolution:
The Payment Method filter cannot be made a mandatory field. As a work around the issue, you can create an expense sheet user defined field (UDF) by name Payment Method with the same options in the drop-down list as the standard field. Make this UDF a required field. After this, you can disable the standard field and have the users enable the UDF by clicking the Settings button on their expense sheets. You can also create an expense sheet disclaimer alerting the users to enable the UDF by clicking the Settings button and to submit their expense sheets after selecting the appropriate payment method.