How to I use custom subdomain in GitHub Pages?

I am getting this error when I try to use a custom subdomain. I have added the CName in my domain provider. But I am getting this error:

The site is live at blog.navendu.me

I also have www.navendu.me which is working fine.

I would like to remove this error and use https encryption on my website blog.navendu.me Is there any way?