Thank you for writing in, sorry that you’re having that issue, that usually caused by an incompatible version of X and Cornerstone.
Please make sure your X and Cornerstone are in the latest version (7.1.2 & 4.1.2 respectively), check your X version first if it’s not 7.1.2 then please reinstall it (since you cannot do the automatic update). You can download the latest version of X here.
If it’s already up to date then more likely it’s your Cornerstone that is not updated, to easily update the Cornerstone, please navigate to plugins panel, deactivate and delete the Cornerstone, once you delete the Cornerstone, there will be a notice on top of the admin dashboard to re-install the Cornerstone, follow that notice and that should install the latest version of Cornerstone.
After you successfully update both X and Cornerstone, try to revalidate your site again.
If the issue persists, please provide us login credentials in a secure note so we can take a closer look.