Hi there,
Thanks for writing in! You don’t need to validate Cornerstone plugin and you only need to validate your X theme.
If you’re getting this error with regard to Cornerstone, probably your X theme is corrupted or you’re not using the latest version of X theme or Cornerstone. Please check the version numbers from here and follow our update guide here to update if necessary.
In case if you’re fully updated, try re-installing your X theme again and it should resolve this issue. You can switch to a different theme temporarily (default wordpress theme), then delete X theme and then you can re-install X theme. All the settings will be saved on the database, so that you won’t lose your data.
Hope that helps.
Thanks!