"The woman is under arrest."
Übersetzung:Die Frau befindet sich in Gewahrsam.
It should be. In most movies where the suspect is arrested the sentence: "You are under arrest!" is translated with "Du bist/Sie sind verhaftet!". It would be the first translation coming to the mind. Of course that is a more idiomatic look on the sentence, duolingo seems to view it more literally. "the arrest" = "der (Polizei)Gewahrsam/der Arrest/die Haft". But cut them some slack there are like a hundred ways to say this. So reporting, reporting, reporting.
I hope I could be of help.