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