You are receiving the below error message when trying to publish an invoice.
Invalid or missing Project Reference.
This error will appear if you have not selected a Project code from the dropdown menu on the invoice. Please note that if you have selected a Cost code on a Purchase invoice, Sage 50 requires you to also select a Project Ref code.
To resolve this, please do the following:
Unpublish the affected invoice(s).
Select a Project Ref code from the dropdown menu.
Republish the invoice.