Scenario:
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"

Cause:
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.

Resolution:
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.