Unable to enable SSL on subdomain

My site is hosted via GitHub page, and recently, I added a subdomain, but the option to Enforce HTTPS is unavailable.

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

I’ve added the following record -

Screenshot from 2020-05-19 06-21-41.png

PS: The current repository for the subdomain is private.

What am I missing? I’m doing something wrong? If not, how to enable SSL on the subdomain? 

Thank You.

This is possibly an issue on your Domains’ end as GitHub tells it to you (You even quoted that message).
So there is little that can be done on GitHub’s end I suppose and you need to check if your DNS/Domain Settings are setup properly to use SSL.
Does your domain even use SSL? When the domain defaults to http:// then it’s a no and you need to setup the domain to always default to https:// with a proper SSL certificate.