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Let's try some steps to get it working, smeinha.
We have several troubleshooting that can help resolve the issue. Make sure to take the exam after every steps to check if it's already working.
This is a possible browser issue. Let's start by using a different one when taking the exam. A complete list of supported browsers is discussed here: Supported Web Browsers For ProAdvisor Certification Course.
Aside from that, you also need to enable or allow pop-ups. Follow the steps in this article depending on the browser that you're using: Enable Pop-ups.
In addition, you may need to clear cache or delete temporary internet files. Let me share this article on how to do that: How Do I Clear My Browser Cache And Temporary Internet Files?
Let me know if you have other questions. I can get back here anytime to make sure everything is working out.