QuickBooks Desktop Sync Error - "Bill External GUID" field has an invalid value " Some Number." QuickBooks error message: A transaction's external guid must be unique.

Cause

You've received this error as there is an invoice in your Quickbooks Desktop with the same combination of Invoice Number and Vendor OR the invoice you are trying to import has already been imported into QuickBooks. The error is to flag this invoice as a possible duplicate. 

Resolution

You should confirm that the invoice is in fact a duplicate invoice. If you find the invoice in QB is the same invoice, you can simply "Mark the Invoice as Exported" in Beanworks. Since the data is already in QB, you don't need to export it again.

It's possible that the invoice is actually a different invoice, this sometimes happens with vendors that are poor at using unique invoice numbers.
In this scenario you should reset the invoice in Beanworks and amend the invoice number to something unique. Once you export and sync the invoice again the invoice will populate in Quickbooks as it no longer views it as a duplicate.