When I access my site (sigma-excels.net) I get a Not Secure comment in the URL field but the site is still displayed okay.
When I access the site with https://www.sigma-excels.net all browsers give me a Not Secure window and all sorts of warnings about the site may be bogus and the certificate is not valid.
Is there something I have to do to remedy this situation?

There is no SSL certificate for your site as https. Therefore that is not valid. The warnings are correct. Only the http://yoursitename should be used.

CC was supposed to give an option for sites with domain names. It's been a long time since that statement was made.

I'm guessing the "duplicate" issue consumed a lot of their resources. There are several older topics about this. If you don't have a domain name, I wrote a script that makes a site secure