https://userapps.support.sap.com/sap/support/knowledge/en/2955754
2955754 - Output message M8 307 for non PO invoice - SAP ERP & SAP S/4HANA | SAP Knowledge Base...
A message output for a non-PO invoice, which posts directly to G/L accounts, is required. During the message processing, error M8 307 is being encountered.