Just check this link(Unit 1 - Player Control - Unity Learn), it is not working and it is link from basic course from Unity
Yep, not working. It seems to be failing to fill the iframe they are using to display the pages content. Tried deleting the cache, cookies etc nothing. It seems to be more terminal than that. Probably wise to raise it with unity and let them sort it out.
It may well be that they are in the middle of deprecating that page and replacing it with new content.