cac:TaxRepresentativeParty/cac:PostalAddress[BR-20]-The Seller tax representative postal address (BG-12) shall contain a Tax representative country code (BT-69), if the Seller (BG-4) has a Seller tax representative party (BG-11).
Hitting this rule in production? Paste the invoice into the validator or wire the API into CI so it never ships broken again.