You do not need to purchase Cornerstone as you already have the X theme. But it seems that the version of the theme is very old that you see the message.
Please, kindly backup your website first and then update the theme to the latest version. Then deactivate and delete the Cornerstone plugin.
After that follow these steps to validate the theme.
Then go to the plugins menu in the WordPress dashboard and you will see that Cornerstone is installed automatically.