Most probably you're using an older version of X theme or Cornerstone. Please check your version numbers with the latest versions listed here (https://theme.co/apex/forum/t/troubleshooting-version-compatibility/195).
If you're using an older version, you need to download the latest X theme from your Envato account and then update manually. You can follow our guide here (https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62).
Top update Cornerstone, you can simply delete the current version from the plugins section and then head over to X -> Overview section and wait for few seconds to install it automatically.
Once you confirm the latest versions, try validating your site again and see if that helps.