The project selection can be made mandatory on expense sheets. This feature can be enabled/disabled in permissions, this would require to edit the permission assigned to a user for whom the project selection needs to be made mandatory on expense sheets.

Follow the steps given below:

Log into Web TimeSheet with administrative rights.
Click on the Administration icon at the top menu.
Click Permissions in the left hand menu, under in the Users/Department category.
From the list of permissions, if you want to modify an existing permission click the Edit button beside the required permission.
On the edit permissions page expand the Expenses tree, by clicking on the '+' sign beside it.
Select Project Specific from the drop-down beside Enter Expenses.
Click Save.