Thank you for writing in, I have check the site that is associated on your license and I see it has an outdated version of X theme and incompatible version of Cornerstone (
X 5.2.2 / Cornerstone 3.3.8).
To resolve this please update both to the latest versions (
X 7.0.4 / Cornerstone 4.0.4) by following the guide on the link below. Since your X theme is outdated (more likely) the Automatic Update method will not work, so please follow the Manual Updates instead.
Updating Your Themes and Plugins
Remember to clear all your caching features (plugin, server-side, CDN, and browser's cache) after updating so that the code from the latest release is always in use. This will help you to avoid any potential errors.
After a successful updating, please try to validate your site again.
If you're having trouble updating the Theme, please provide us login credentials (/wp-admin and FTP) in a secure note so we can take a closer look.