If you see a 404 error after clicking on the link, this is almost certainly because you are already logged into another Google account. Log out of that account and try again. You will then log in with your Treehouse Google account (email@example.com).
The only valid addresses for the public website (which you are currently on) is https://www.treehouse-kids.de, so if you are providing someone with the web address of Treehouse use this.
The address https://treehouse-kids.de/ does not work!