DE-R-020
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-020
When both seller and buyer are located in Germany and the payment means code (BT-81) is 59 (SEPA direct debit), the debited account identifier (BT-91) must be a valid IBAN.
How to fix it
Check the payer account cbc:ID – IBAN without spaces and with a correct check digit.