The "The SSL certificate for this website is not trusted" error message is typically encountered when a user tries to access a website that is using an SSL/TLS certificate that is not trusted by the user's browser or operating system.

When a browser connects to a website using HTTPS, it checks the SSL/TLS certificate presented by the server to verify that it is valid and trusted. If the certificate is not trusted, the browser will display a warning message to the user indicating that the certificate is not trusted.

This error can be caused by a number of issues, including:

To resolve this issue, you should check that the certificate is issued by a recognized CA, that it is not expired or not yet valid, and that the certificate chain is complete. You can also check the certificate revocation list (CRL) to make sure the certificate has not been revoked by the issuing CA.

If the certificate is self-signed, you can either obtain a certificate from a trusted certificate authority (CA) or you can add the self-signed certificate to the trusted root certificate store on the user's device. However, adding a self-signed certificate to the trusted root certificate store may not be a secure option as it will be trusted by all applications on the device, not just the web browser.

It is also important to note that some browsers may have stricter security policies, which will result in more errors on untrusted certificates.

To avoid this error in the future, you should obtain a certificate from a reputable CA and make sure that the certificate is properly configured and installed on the server. Also, you should keep track of the expiration date of the certificate and renew it before it expires.

You should also ensure that the certificate chain is complete, and that all the intermediate certificates are installed on the server.

It is also recommended to have a regular testing and monitoring of the SSL certificate to avoid unexpected errors.