It doesn't sound weird to me. If you were incredulous at how strange it was that there was not a mall, you would say, "not a single mall" or "not one single mall", but just stating the fact that there is not a mall in the city doesn't strike me as odd.
"Where's the mall?" "There's not a mall in this city." "You're telling me that there's not a single mall here?!"
Something like that.
I keep reporting a couple of instances in each lesson during this Danish course; The "mapping" between Danish and English is generally less good than all other lingos I have testes @DuoLingo. I'm a native Norwegian (very close to Danish) and like to believe that my English is also decent. In spite of this, I keep getting lots of errors simply because the English translation is often very literal (unnatural) and because there are very few answers accepted for each exercise in the Danish course.
In this exercise, I would instinctively say "no mall in my town" rather than "not a mall". The latter seems natural to me only if emphasising by adding 'single' or similar --> "not a single mall".
Also, the word 'any' is generally disallowed by Duo despite sounding more natural in most cases than 'a'. (Example: 'He didn't bring any money.')
Finally, the course is confused as to whether sg. or pl. is to be used in English when negating a sentence (mix of "there's not a mall" vs. "there's no mall[s]" vs. there's not any apple[s] left"). The same confusion applies to nouns that only has one form in English, e.g. 'a series' and 'several aircraft'.
Please keep reporting -- that's how the courses get better! :-)
Agree that the American bias is annoying, but I have received loads of confirmation emails saying "thanks for your suggestion -- it's now implemented", so I can confirm that somebody is indeed appreciating your reports! Keep it coming, reporting (as long as you're pretty confident you're right about the suggestion) is a great contribution to the community! :-)
The DL sentence is inexact and would not be used in this context. In Denmark you would say "indkøbscenter" (=shopping mall) in order not to confuse it with the many other kinds of "centers", i.e. medical centers, historical centers and, of course, center of the city, which we call centrum.