When registration first opens, the system does get slammed with many users trying to access it at the same time. Slowness and extended loading times are expected, and the only thing to do is be patient, try again, and wait.
At other times there is almost nothing technological that could be wrong with the class registration system that would affect only select individuals. If it's working for most everyone else, it's not a technology problem on the system side.
Since it's a web application, you can do troubleshooting for your web browser. Try incognito or private browsing (if you have any extensions, make sure they are not enabled in incognito/private browsing). Try a different web browser (Chrome, Firefox, Edge, Safari). Clear browsing data, then completely close the browser and reopen it. Try a different device.
If you are connected to the Internet by a non-ACU network, you can also try connecting to a different network.
If these troubleshooting steps do not resolve the issue, the cause is most likely something for which you will need to contact your adviser or the Office of the Registrar.