This error occurs while you are using a self-signed certificate, it is advisable to go through the trusted third party certificates.

You can use SSL installation checker to diagnosis troubleshoots.

https://www.ssl2buy.com/wiki/ssl-installation-checker/

https://www.sslshopper.com/ssl-checker.html

https://ssltools.websecurity.symantec.com/checker/views/cert...