BR-21: A line has no identifier
This code shows up when an electronic invoice is validated. Here is what it means, why it fires, and what to change in the file.
Why this code appears
An invoice line has no identifier (BT-126). It is what lets your customer refer to "line 3" in a dispute.
How to fix it
Number the lines in AssociatedDocumentLineDocument/LineID: 1, 2, 3. Identifiers must be unique within the document.
Where it happens
This code comes from EN 16931, the European e-invoicing standard. It concerns business term BT-126.