FORMAT-CII
A structural check before the rule layers – the file must first be readable as a valid XML document.
What this error means
FORMAT-CII
The invoice uses the UN/CEFACT CII syntax (ZUGFeRD/Factur-X, the CII variant of XRechnung). This validator currently checks UBL invoices only.
How to fix it
Export the invoice as UBL (Peppol BIS Billing 3.0). CII support is planned.