PEPPOL-EN16931-R003: The invoice has neither a buyer reference (BT-10) nor a purchase order…
What it means
The invoice has neither a buyer reference (BT-10) nor a purchase order reference (BT-13) -- Peppol requires at least one so the buyer's system can route the invoice.
How to fix it
Add BT-10 (buyer reference) or BT-13 (purchase order reference), whichever the buyer supplied when placing the order.