This may be a problem with the network or your cdn settings. It is also possible that the website is too loaded to open for the time being. It will be better after the rush hour.

As for how to take the exam, here is an introduction.

You should register first, attend the training, and then arrange the exam after the training. But you can take a mock exam first. The specific method is: click on the website I sent you and drop it down under the interface of the home page. The bottom part has the words "mock exam" on the right. Click on it to take the mock exam.