What this is about
The buyer reference is missing (BT-10). Authorities reject invoices without it — that is where your client’s routing ID (Leitweg-ID) goes.
What to do
Enter your client’s routing ID in the “buyer reference” field. You get it from the client.
Does the error keep coming back?
Then it is not chance but a pipe connected the wrong way. The free 15-minute check finds the spot.
Arrange a check