The Cl@ve credentials have an associated security level that depends on how they were registered in the service. If the level is not adequate, the following error appears:

Error (no. message.validation.error.code) processing request : message.validation.error.code - Assertion is null, empty or the response is encrypted and decryption is not active.

To modify the security level of your Cl@ve credentials, please follow the instructions on this page:

https://sede.agenciatributaria.gob.es/Sede/ayuda/consultas-informaticas/firma-digital-sistema-clave-pin-tecnica/modificar-datos-renunciar-servicio-clave/obtener-nivel-seguridad-superior-certificado.html

Please note that with Cl@ve you can access the site but you will not be able to carry out electronic signing if the procedure requires it.