Hi,
Unfortunately Web Preview for Azure Cloud Shell seems to be broken. This is why step#10 is failing for you.
I'm working on typing up steps for workaround that uses ngrok instead. It is third party utility that does same thing as step #10. It is free, you just have to sign up using name and email in order to get key. If you would like to quickly sign up and get your auth key I can walk you through it right now before I have all the screenshots/steps ready.
If you don't want to use third party tool then I recommend reading over the exercise and making sure you understand what each step is supposed to do for learning purposes. If you have any questions in this regard add a comment below and I would be happy to explain what is going on in each step if it were actually working properly.
-TP