PEPPOL-EN16931-R003
An official OpenPeppol BIS Billing 3.0 rule – checked for invoices sent over the Peppol network on top of the EN 16931 rules.
What this error means
PEPPOL-EN16931-R003
Peppol requires a buyer reference or a purchase order reference — receivers use it to route and match the invoice.
How to fix it
Add cbc:BuyerReference (ask the buyer what to put there) or an order reference (cac:OrderReference/cbc:ID).