Let's Encrypt SSL cand subdomain

I have a domain in https
I created a a subdomain but when I try to create a ssl certificat I have this element

Code:

  File "/usr/lib/python2.7/dist-packages/certbot/client.py", line 253, in obtain_certificate
  self.config.allow_subset_of_names)
  File "/usr/lib/python2.7/dist-packages/certbot/auth_handler.py", line 78, in get_authorizations
  self._respond(resp, best_effort)
  File "/usr/lib/python2.7/dist-packages/certbot/auth_handler.py", line 135, in _respond
  self._poll_challenges(chall_update, best_effort)
  File "/usr/lib/python2.7/dist-packages/certbot/auth_handler.py", line 199, in _poll_challenges
  raise errors.FailedChallenges(all_failed_achalls)
FailedChallenges: Failed authorization procedure. www.demo.xxxxxxxxx.org (http-01): urn:acme:error:connection :: The server could not connect to the client to verify the domain :: DNS problem: NXDOMAIN looking up A for www.demo.xxxxxxxxx.org

I have no registration about this element www.demo.xxxxxxxxx.org who should be demo.xxxxxxxxx.org (demo.xxxx.org)
I just add in my main domain (dns) the element about subdomain. Note the subdomain work perfertly before the ssl certificate sub domain creation

Any idea ?

 

Ok the ssl work now but when I wrtie the subdomain demo.xxxxxxxxx.org it go always on the domain : xxxxxxxxx.org