Your questions intrigued me @eliliang, so I made an attempt to answer them somehow. You'll find my response to your questions via the link below:
If you or anyone else takes the time to read it, hope you find it useful.
The usual English syntax would be "There are no people outside".
But I'm not certain whether Duo accepts "outside/outdoors" in this particular exercise, even though both are accepted in other exercises. If not accepted, then it needs reporting by users and then correcting by Duo.