The debtor bank is not registered under this BIC in the Sepa direct debit system.
Debtor bank/ Creditor: The accessibility of the debtor bank was not checked when accepting or creating the files.
Ask the party that sent the error message to do a two-step verification to verify that the checks are correct. Contact the debtor to agree on a different payment method and inform him that his bank does not support direct debits. When creating the mandate, use a tool (e.g. Twikey) that does this verification automatically. This way you avoid wasting time for both you and your customer.