FCX-05: The embedded CII document's guideline identifier…
What it means
The embedded CII document's guideline identifier (GuidelineSpecifiedDocumentContextParameter/ID) is either missing or does not match any of the five known Factur-X/ZUGFeRD profiles (minimum, basic WL, basic, EN16931, extended), so the invoice's profile could not be determined.
How to fix it
Set GuidelineSpecifiedDocumentContextParameter/ID to the exact URN of the intended Factur-X profile, e.g. 'urn:cen.eu:en16931:2017' for EN16931/COMFORT.