To install a free SSL on a website hosted on your cPanel plan, navigate to the Security section and click on Lets Encrypt™ SSL:

Click on Issue next to your domain name. If it’s not the main domain, make sure you have it as an addon of the parked domain in your cPanel:

Select the http-01 validation method and click Issue:

After issuing the certificate, you will get a notification that the SSL Let’s Encrypt has been successfully installed.