I don’t know why you try to use a cname to validate your domain, could you please elaborate how are you trying to validate it?.
Depending how you are trying to validate your domain maybe it could be possible to cname that _acme-challenge for your domain to another host but if you are doing it manually I see no obvious reason to not create a TXT record for your domain with the provided token.
Oh in ssl for free if you want to order wildcard, you need the dns manual verification and there is no other options to validate the domain if you want to order wildcard.
Oh, I was wrong here...
Cname is actually different by txt dns I think...