This error can occur for two reasons:
Invoices processed and supplier account selected before integrating with Xero
Invoices processed and supplier account selected while connected to another accounts package then switching integration to Xero.
While the description in the drop down menus might be the same the code in the background is different for each accounting package producing this error.
To clear the error and process the invoices follow these steps:
Unpublish the affected invoices
Clear the Customer/Category/Tax codes
Select Supplier/Category/Tax codes again
Publish invoice again
Please note: The error message will not immediately disappear. You will need to refresh the page and Invoice will then publish!