Issue: Processed/completed invoices won't sync over to MYOB. When you check the error logs, the above error, or something similar comes up.
Explanation: The most important part of this error is the "Field Not Found" message. If the Additional Details looks similar (doesn't have to have the word "purchase"), then it is the same resolution.
Resolution: The problem here is that an invoice with the same reference number was deleted from MYOB, but wasn't voided in Workshop Software, or vice versa. Please make sure that both sides are voided/deleted before creating the new invoice to sync over.