FCX-02: The embedded invoice file's Filespec dictionary has no /AFRelationship…
What it means
The embedded invoice file's Filespec dictionary has no /AFRelationship entry, so PDF readers cannot tell how the attachment relates to the visible document (e.g. that it is the machine-readable source of the invoice).
How to fix it
Add /AFRelationship /Data (or /Source, per the Factur-X profile) to the embedded file's Filespec dictionary.