BR-01: The invoice does not say which specification it follows (BT-24, Specification…
What it means
The invoice does not say which specification it follows (BT-24, Specification identifier), so software cannot tell which rule set to validate it against.
How to fix it
Set BT-24 (CustomizationID) to the exact profile URN your recipient expects, e.g. the EN16931 or XRechnung URN.