The currency code (BT-5) is not an active ISO 4217 code — either a typo or a withdrawn code (e.g. CUC, HRK).
Use a current ISO 4217 code (e.g. EUR); replace withdrawn codes with the successor currency.
The Billhorse validator checks XRechnung, ZUGFeRD and Factur-X right in your browser — your file is never uploaded.
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Rule source: EN 16931 / XRechnung (KOSIT) or Billhorse check (BH-*).