Your service provided is not a “qualified” one or the seal it provided is not qualified.
Please request the seal to a QSTP a trust service type. Of the different services offered by qualified trust services, you should request a “Qualified certificate for electronic seal”. A list is available from the “EU Trust Services Dashboard”.
The Seal has to be compliant with the eIDAS legislation.
Other common reason for rejection is that the uploaded document is not sealed with a “Qualified seal” but e.g. with an “electronic signature”.
Disclaimer: Please note that the European Commission cannot provide a legally binding interpretation of the EU legislation, as this is the sole competence of the European Court of Justice. Any remarks from the European Commission services are without prejudice to the position the Commission might take should related cases arise in a procedure before the Court of Justice.