Hi Sarah,
Thanks for writing in! Probably your current theme is corrupted or you’re using an older version of X theme and Cornerstone plugin.
First delete your Cornerstone plugin by head over to your WordPress plugins. Then head over to Appearance -> Themes section, switch to a different theme (default WordPress theme) and then delete your X theme as well. Then download the latest version of X theme from your Envato account and re-install it. This will automatically install Cornerstone plugin as well.
Then you should see Validation section under your X menu. If you’re still having issues, please disable your 3rd party plugins as well and see if you’re experiencing a conflict with your 3rd party plugin.
In case if you require further assistance, please provide us with your WordPress login credentials in a secure note.
Thanks!