Please ensure you are running the course on a laptop or PC and using either Edge or Chrome as the preferred browser, they provide the best user experience. Please also ensure that your internet connection is good enough.
Also if required you can clean the cookies and cache and then restart your computer. Below are the links to the instructions on how to clean the cookies & cache.
For MAC and Safari browser user
https://www.businessinsider.com/how-to-clear-cache-on-safari-browser
For Edge User
https://support.microsoft.com/en-us/search?query=edge%20clear%20cookies%20and%20cache
For Chrome user
https://clear-my-cache.com/en/windows/google-chrome.html