Hello,
I am sorry that you wish to cancel the W3 Total Cache Pro license.
I am not sure, what the email suggests but I am sure that W3 Total Cache is compatible with the Siteground server, you just need to configure the plugin to respect their Dynamic Caching or disable it so that W3 Total Cache can fully take responsibility for caching.
Since this is a public forum, please reach out to us via the Plugin in Performance>Support or via the website contact form and share your license details.
Thanks!