Issue: If the synching errors Customer Cannot be changed displays (as seen below),this is why:
Explanation: The customer name has been changed for that invoice in M.D (I.E from Customer A to Customer B) and MYOB rejects the invoice with a completely different customer name (now Customer B). This causes the error.
Resolution: To resolve this, they must change the Customer A name to Customer B in MYOB and then sync again.