Alias , due to the fact that it is not a physically added website at the server level, but only a redirect rule (at the server level), does not have the ability to install or generate an SSL certificate. Regardless of the option, i.e. whether it would be a commercial certificate or a free Let’s Encrypt.
If you plan to create a redirect from one address to another and a secure connection is a requirement, it is best to add a new blank page in the Websites section of the mybox panel.
And then create a .htaccess file in its root directory (public_html by default) that will contain the redirect rules.
We have described this type of action HERE.