CPanel will attempt to install a free SSL certificates for domains and subdomains added in your CPanel that pass a Domain Control Validation (DCV) test, which proves ownership of the domain.
Auto-SSL has a limit of 200 domains/subdomains.
To see if a certificate has already been installed for your domain try connecting to it using https:// instead of http://
Please note that it can take some time (~48 hrs) for the Auto-SSL certificates to be installed after a new domain or subdomain is added in your CPanel.
To redirect all connections to use https:// instead of http:// please see this htaccess method of redirecting to https in our Knowledgebase.