Based upon the error message, you are missing the invoice format code in the Invoice and Adjustment Maintenance screen. Pull up that project / draft number and check here (circled). Define the invoice type, save and then try again.
This is usually populated based upon the default value in 'Billing Information Maintenance' screen, so perhaps that got missed in the setup. I would also set the default value there for this project so it is defaulted for the next invoice.