@gsmith @ttsandrew I have found the solution and now successfully completed the HTTPS setup.
The browser is showing ‘Not secure’ because its a self-signed certificate and new versions of browsers will show it like this.
I have made below change in ‘openssl-graylog.cnf’ file. Without this change browsers were throwing SSL key errors as shown in earlier messages.
I would suggest making change in Graylog official documentation and update this with above change.
Thanks a lot @gsmith and @ttsandrew for your instant responses and helping me out.
Hopefully if anyone else face similar issues , this post will be helpful.
Thanks!