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:
Google Chrome v60 or later
Mozilla Firefox v60 or later
Safari v11 or later
Internet Explorer v11 or later
Microsoft Edge v14 or later
Opera v50 or later
If you have IE 10 or less on Windows 7, the browser supports this new security, but it is turned off by default. To turn it on, Click the Tools menu, then click Internet Options. Click the Advanced tab and scroll down in the checkbox list to find the Security section. Make sure there is a checked box beside Use TLSv1.1 and Use TLSv1.2.
2. Delete your cookies and refresh your browser
3. Clear your cache and refresh your browser
4. Try to register on another device