Hi there,
You can only install other bundled plugins once you validated it. Or, you can download them from your ThemeForest along with the theme.
And you should validate your theme and not the cornerstone because your license is only applicable for the theme. Cornerstone is just a bundled plugin that comes with it. So, the cornerstone should appear validated once you validate your theme.
You should check your theme installation to make sure that cornerstone won’t need re-validation. Example, you downloaded the zip package from ThemeForest and install x_package.zip directly instead of installing x.zip package alone, which turns out the installation folder to be /wp-content/themes/x_package/x/, that’s wrong. The correct installation folder should be `/wp-content/themes/x/, you should correct it through FTP, or your hosting panel’s file manager.
You may provide your site’s URL so I can confirm if your folder installation is correct.
Thanks!