PEPPOL-EN16931-R051: An amount's currencyID attribute does not match the invoice currency code…
What it means
An amount's currencyID attribute does not match the invoice currency code (BT-5), except where the VAT accounting currency is explicitly allowed.
How to fix it
Set every amount's currencyID to the same value as BT-5 (or the VAT accounting currency where that is explicitly allowed).