Hi Sulli,
The SSL cert can take a little bit to provision as it's dependent on LetsEncrypt being able to resolve the host before issuing the certificate.
Check the DNS propagation and wait a little, then either wait for it to issue, or go to the websites section, click on the relevant cp site, then go to security, click on the 3 dots next to the hostname and click on 'request certificate'.
I hope that helps
Matt