If you have followed everything in our Validation Issues Troubleshooting Guide, the cause is highly likely because you're using a very old version of X and Cornerstone.
In this case, you would need to manually update X and Cornerstone. You can follow the Manual Update instruction here: https://theme.co/apex/forum/t/setup-updating-your-themes-and-plugins/62. Also, update WordPress as the latest versions are made to be compatible with the last 3 versions of WordPress
After everything's updated, try validating again.
If that doesn't help, please provide the following in a Secure Note
- WordPress Login URL
- WP Admin Username and Password
- cPanel access (ask your host if you're not sure)