If you have connected the expense sheet to a project you need to have report codes for project connected to all expense code you use in the expense sheet. Example Other Expense Codes:
Please check all expense codes that are used in your expense sheet and make sure they have a connection to a report code (milage, other expense, entertainment, allowance)
Okey, then you need to check the project activity you used in the expense sheet. Make sure that report code exists on the project activity and that the activity are in released status. :-)