If you are using MVC development model, make sure the presentation URL is correct and the live site is up and running. Then, there is no firewall or other network element blocking the communication between Kentico admin app and the live site app. I would also check the Event log and browser development tools to see if there is more detailed or other error message. If the admin app and live site are running on different domains, you may need to configure SameSite cookie. And also, make sure you are not using custom CSP or other headers in the config files which may be blocking the form builder.