Thank you for writing, Is that the site you're working on below? Your current X (6.0.4) is not compatible with your current Cornerstone (3.1.3), please update both to the latest version X 6.1.4, Cornerstone 3.1.4
Updating Your Themes and Plugins
Remember to clear all caches (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 you have successfully updated to the latest version.
Try to validate your X theme again (X > Validation).
Let us know how it goes,