What does BR-DE-3 mean?

Seller city (BT-37) is required by XRechnung, not by EN 16931. The rule runs under XRechnung 3.0 (UBL).

How do I fix BR-DE-3?

Add Seller city to the invoice. In UBL the element is ⁦/Invoice/cac:AccountingSupplierParty/cac:Party/cac:PostalAddress/cbc:CityName⁩.

XRechnung (BR-DE) rules

Seller city (BT-37) is required by XRechnung, not by EN 16931.

An invoice built like a valid one, except for this:

<cac:PostalAddress>
  <cac:Country>
    <cbc:IdentificationCode>DE</cbc:IdentificationCode>
  </cac:Country>
</cac:PostalAddress>

The validator says: Seller city (BT-37) is required and the document does not contain it.

Add Seller city to the invoice.

What does BR-DE-3 mean?: Seller city (BT-37) is required by XRechnung, not by EN 16931. The rule runs under XRechnung 3.0 (UBL).
How do I fix BR-DE-3?: Add Seller city to the invoice. In UBL the element is ⁦/Invoice/cac:AccountingSupplierParty/cac:Party/cac:PostalAddress/cbc:CityName⁩.

Seller city (BT-37)UBL /Invoice/cac:AccountingSupplierParty/cac:Party/cac:PostalAddress/cbc:CityNameCII /CrossIndustryInvoice/SupplyChainTradeTransaction/ApplicableHeaderTradeAgreement/SellerTradeParty/PostalTradeAddress/CityNameRead from what WorkAlio's CII parser accepts — there is no CII writer yet, so this path is not independently verified the way the UBL path is.