How to fix PDF-21
The embedded XML is ZUGFeRD 1.0, which is not checked
A check of the PDF of a ZUGFeRD or Factur-X invoice, not of its XML. It runs when you upload a PDF; the embedded XML then gets the XSD and EN 16931 checks (XRechnung: also the KoSIT rules). Check your PDF with the ZUGFeRD validator or the Factur-X validator. What the PDF check covers.
What this rule checks
ZUGFeRD 1.0 uses its own XML (CrossIndustryDocument), not the EN 16931 CII syntax (CrossIndustryInvoice), and does not meet EN 16931. It is no longer a valid e-invoice in Germany or France, so the XML checks do not run on it.
How to fix it
Create the invoice as ZUGFeRD 2.3 / Factur-X 1.07 (factur-x.xml) with an EN 16931 based profile.
Related rules
Validate your invoice
Upload your corrected invoice to check it passes PDF-21 and all other ZUGFeRD / Factur-X PDF checks and EN 16931 rules. Free, instant, no signup.