CNAME records

You must own a custom domain to use Cloudflare. You cannot use a free subdomain.
Because you are not the owner of this domain ( - that domain belongs to hosting.

If you really want to have sub/domains via CF (not hosting free subdomain)

one of the free options is to use Freenom
and choose one of the free TLDs (except .tk)

Then for the subdomain read the article from my first RE