HTTPS Full enabled in Cloudflare, but URL still shows "not secure" for GitHub page

I have a custom domain that is setup and I can correctly navigate to the custom domain, which points directly to the GitHub page. However, in the browser it shows that it’s not secure. I have “Full” HTTPS enabled within Cloudflare, but the settings doesn’t allow me to “Enforce HTTPS”. It says the following:

Enforce HTTPS — Unavailable for your site because your domain is not properly configured to support HTTPS

How is the domain not properly configured to support HTTPS if I can navigate to https://mycustomdomain.com and successfully retrieve the contents of the GitHub page? 

I can access the GitHub page through my custom domain using HTTPS, but it’s just not secure. It doesn’t seem like the Cloudflare SSL certificate is being issued in this case, as the Issuer is DigiCert.

I saw some other articles referencing pointing A records to some IP addresses, but I have not been able to discover that anywhere in the documentation, so I haven’t done that. I just simply have a custom domain set up in Cloudflare and two CNAME records, one for www.mydomain.com and one for mydomain.com. The www.mydomain.com points to mydomain.com, which that CNAME record points to mygithubuser.github.io.

Is this one of those cases where I have to wait several hours for this to work properly?

Guess it just took awhile. It’s working now.