created subdomain as a website and letsencrypt cert not creating

Just created a subdomain (using add website rather than add subdomain). Can ping website and can get index page if I override the certificate test. It shows that the site is using the host server certificate.
The SSL and Letsencrypt buttons automatically untick themselves>
I remember having this problem some years ago but can't find the answer.

 

is it also set to auto create a www subdomain to that subdomain address?
i'd assume the dns record for www.subdomain.domain.tld doesn't exist, so it'll fail on that.

 

Well spotted. My error. That's what it was. Now works fine. Thank you!

 

Thank you Till, I'll use that error FAQ next time