"It is cold, take my jacket."
Translation:Es ist kalt, nimm meine Jacke.
Because Mantel is generally more like a coat, overcoat or cloak (i.e. it is larger and heavier than a jacket) so Duo did not include it in the list of correct answers. However I checked my dictionary and it does include "jacket" as one translation of Mantel but only if it is an overcoat type of jacket (whatever that is!). https://www.dict.cc/?s=Jacket So you could have reported it but I think it might have been hard to convince Duo to allow it.