You can access the XML document validation service (UBL 2.1 standard) at the following link:
https://ecosio.com/en/peppol-and-xml-document-validator/
You need to select the following "Rule set": "UBL Invoice 2.1" for invoice validation in the UBL 2.1 standard.
The service allows testing of all relevant document types in the UBL 2.1 standard