The offered SSL certificate will not cover subdomains of an alias domain to avoid high number of requests requiring valid SSL certificates in the web server.
If you want the offered SSL certificate to cover subdomains of an alias domain, you will need to remove the alias domain and then add the domain as an addon domain.
Once complete, you can manually run the SSL certificate installation from cPanel > Security > SSL/TLS Certificates.
For more information about installing the free SSL certificate, please refer to the following article : How to check SSL certificate status in cPanel?