BR-DE-15: XRechnung requires a buyer reference (BT-10, the German 'Leitweg-ID' in…
What it means
XRechnung requires a buyer reference (BT-10, the German 'Leitweg-ID' in public-sector invoicing), which plain EN16931 leaves optional.
How to fix it
Populate BT-10 with the buyer reference the buyer provided (e.g. their Leitweg-ID for German public-sector invoices).