The project you selected is not associated with the customer you selected
Why the sync error occurs
When you select a project dimension on a bill, that project must be associated to the customer dimension that you selected at the line item level, in Sage Intacct, or this error will occur
How to fix the sync error
In Sage Intacct, associate the selected customer to the selected job/project, and sync.
Alternatively, edit the bill and select a customer and job that are already associated in Sage Intacct and sync again.