When a user tries to add an expense sheet in Web TimeSheet, the following error message occurs,

"You do not have permission to create an Expense Sheet"

This error occurs if the user has the Enter expenses that are Project Specific is enabled but the user has only a Time & Attendance seat assigned to him. If the user does not have a Project and Billing seat assigned but in the permission he has enter expenses that are project specific enabled, the above error pops up.

You can resolve it by either assigning the user a Project & Billing seat or by changing the Permission to Enter expenses to both, Project Specific and those that are not Project Specific.