Please try opening the lesson using the latest version of Chrome which you can get from https://www.google.com/chrome/browser/desktop/index.html.
For mobile phone users, it is recommended that you use the Puffin Browser which you can get from the following:
Android: Google Play (https://play.google.com/store/apps/details?id=com.cloudmosa.puffinFree&hl=en&gl=US)
iOs: Google Play (https://apps.apple.com/us/app/puffin-web-browser/id472937654)