DE-R-015
A Peppol Germany national rule (DE-R-*) – applies when the invoice is sent over the Peppol network to a receiver in Germany.
What this error means
DE-R-015
When both seller and buyer are located in Germany, the buyer reference (BT-10; Leitweg-ID in German B2G) must be provided.
How to fix it
Fill in cbc:BuyerReference – ask the buyer for the expected value.