KSEF-NIP
A check of the Polish KSeF FA(3) system – applies to invoices submitted to KSeF; a separate format, not Peppol.
What this error means
KSEF-NIP
A NIP (Polish tax number) in the invoice fails its checksum — most likely a typo. KSeF rejects invoices with invalid NIPs.
How to fix it
Compare the 10 digits of the NIP with the registration documents and correct the typo.