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).
What the rule checks
As the rule set's publisher wrote it. The rule runs on every element the context matches, and fails where the test is false; the validator reports the line of that element.
cac:TaxRepresentativeParty/cac:PostalAddress
normalize-space(cac:Country/cbc:IdentificationCode) != ''
Checked with EN 16931 Invoice (UBL), EN 16931 Credit Note (UBL), Croatia eRačun Invoice (UBL), Croatia eRačun Credit Note (UBL)
//ram:SellerTaxRepresentativeTradeParty
normalize-space(ram:PostalTradeAddress/ram:CountryID) != ''
Checked with EN 16931 Invoice (CII)
cac:TaxRepresentativeParty/cac:PostalAddress
(cac:Country/cbc:IdentificationCode) != ''
Checked with Romania RO e-Factura Invoice (UBL), Romania RO e-Factura Credit Note (UBL)