BR-CO-09: The seller VAT identifier (BT-31), seller tax representative VAT identifier…
What it means
The seller VAT identifier (BT-31), seller tax representative VAT identifier (BT-63), or buyer VAT identifier (BT-48) does not start with a valid ISO 3166-1 country prefix (Greece may use 'EL').
How to fix it
Restore the two-letter country prefix on the VAT identifier at the source system. This touches a party identity, so it is reported, never auto-repaired.