If you log on to Duolingo online, and add the page to your home screen, you can access the full version of the notes so you can learn the topic before attempting the lessons. This is better than the actual app, as the online, web version of Duolingo offers the full version of notes & tips.
Yeah, that's a perfectly fine answer, I'm not sure what to tell you. My best guess is you just had a typo or something, but it could be a glitch too. Next time you come across an issue like this, make a screenshot and link to it; that'll help to diagnose the problem.
It's either "Geh(e) nicht durch die Tür" (informal, addressing one person) or "Gehen Sie nicht durch die Tür" (formal, addressing one or more people).
How to understand the placement of nicht here?
I read somewhere else that when nicht is placed at the end of a sentence, it negates the entire expression. So to write this sentence in non-imperative form, it would be Du gehst durch die Tür nicht. Following this logic, is it correct to say Gehe durch die Tür nicht?