Why web browser showing certificate not trusted error ?
The most typical cause of a “certificate not trusted” problem is because the certificate installation on the server (or servers) hosting the site was not finished properly. To check for this problem, use our SSL Certificate tester. An incomplete installation in the tester results in one certificate file and a broken red chain.
https://www.digicert.com/kb/ssl-support/certificate-not-trusted-error.htm