The rule requires that a "Buyer reference" (BT-10) element is present and contains a non-empty value. The test checks that the element exists and that its content is not empty or only whitespace. A document fails when this element is missing or contains only whitespace.
How to fix
Add a non-empty cbc:BuyerReference element in UBL invoices or credit notes, or add a non-empty ram:BuyerReference element under rsm:SupplyChainTradeTransaction/ram:ApplicableHeaderTradeAgreement in CII. Provide the buyer's reference value as the element content.
Guidance on this rule. The original text above is authoritative.