How to add domain security certification, in WordPress website?

“Not Secure” warning refers to the lack of security for the connection to the page that you are visiting. It suggests that the information sent and received with that page can be exploited by the hackers.

Have you done the follow things?

  1. Getting an SSL certificate
  2. Installing your certificate through your web host
  3. Changing your WordPress URL
  4. Implementing a site-wide 301 redirect

If the answer is yes, there might be an issue with the cache. Try clearing it. It should fix your issue.