If you are getting an error, try these "fixes," in this order, before calling the testing center. The number is on your registration receipt as well as the website.
1. Make sure your browser is a current version of a modern browser. Some common browsers include Google Chrome, Mozilla Firefox, Safari, Microsoft Edge, and Opera.
2. Delete your cookies and refresh your browser.
3. Clear your cache and refresh your browser.
4. Try to register on another device.