Hi Iban, welcome to the Community!
This error:
Tells that your SonarQube server is configured with HTTPS and a custom SSL certificate. However the certificate is not correctly configured in the scanner machine’s JVM. This configuration is outside of SonarQube scope.
Java is basically telling you that the server certificate is unknown and could not be validated with the provided truststore. What you have to do is add the SonarQube server certificate to the Java truststore. This Oracle documentation gives a good overview of this.
Cheers,
Daniel