FCX-01: The hybrid PDF's XMP metadata does not declare pdfaid:part=3, so the file…
What it means
The hybrid PDF's XMP metadata does not declare pdfaid:part=3, so the file cannot be identified as PDF/A-3 -- the container format Factur-X/ZUGFeRD requires for the embedded invoice XML to be considered part of the same conformant document.
How to fix it
Set the XMP pdfaid:part property to 3 (and pdfaid:conformance to a valid PDF/A-3 level) when generating the PDF.