For cPanel, there is a feature called AutoSSL which is able to get and install Let’s Encrypt certificates for users.
Your hosting provider needs to have this feature installed and available, and only the hosting provider can choose and arrange to do this (there’s nothing that you or we can do to enable it if they don’t). There are other options if the hosting provider for some reason doesn’t support AutoSSL but does allow you to import a certificate obtained elsewhere, but hopefully that’s not the case because it’s significantly more work in comparison to AutoSSL.
According to the thread at
another Site5 customer was previously told that Site5 would not support Let’s Encrypt at all (apparently as a deliberate policy decision). Perhaps you could check with their support about whether this information is correct and up-to-date.