Hi There @riasage
Thanks for writing in! First of all, please go through our builder preview troubleshooting guide here (https://theme.co/apex/forum/t/troubleshooting-builder-preview-warnings/14404) which has possible issues and solutions.
Also make sure that you're running PHP v5.6.x or later on your server to avoid incompatibility issues.
If you're still having issues, please provide us with your WordPress login credentials in a secure note to check your issue further.