Hey Amaravati,
Thanks for writing in!
When Cornerstone is not automatically installed after installing the X theme, you may have permission issues. You can get it installed again by going to X > Validation and wait for a few moments while it attempts to install Cornerstone. You should be seeing a message that the Cornerstone plugin has been installed or not.
Now, if you want to install it manually, you should be able to download it within your site. For example, if your site is https://example.com
, Cornerstone should be located in this URL: https://example.com/wp-content/themes/x/framework/cornerstone.zip
.
Hope this will help you. Kindly let us know how it goes.