I have a registered domain - https://mentalhealth-kb.net and added a Let’s Encrypt certificate to it last week.
The GitHub Pages URL for it https://mhwkb.github.io/
The first problem is that in the options for my repo, under the custom domain section, the check box is disabled (after I enter in the the custom domain and save).
Enforce HTTPS — Unavailable for your site because your domain is not properly configured to support HTTPS ([mentalhealth-kb.net](http://mentalhealth-kb.net/))
The second problem is that when I go to https://mentalhealth-kb.net in my browser, I get the message
Your connection is not secure mentalhealth-kb.net uses an invalid security certificate. The certificate is only valid for the following names: \*.github.com, github.com, \*.github.io, github.io