Fixture ID: emptyXRechnung · XPath: /ubl:Invoice/cbc:BuyerReference
XRechnung requires a buyer reference (BT-10). For German public-sector buyers this is the Leitweg-ID; business buyers may supply any reference, but the field must be present.
BR-DE-15Ask your client for their Leitweg-ID (public sector) or an order/customer reference, and set buyerReference.
"buyerReference": "04011000-1234512345-06"