Translation:They are the last animals and the last trees.
It probably did not accept it because ""They are last animals and last trees" is incorrect English/wrong English grammar.
It needs to be ""They are the last animals and the last trees".
Or at least have one "the' as in "They are the last animals and last trees." Although when using only one "the" it really would be better, more correct, to leave out the second "last" as well, as in "They are the last animals and trees"
The meaning is slightly different in between the two sentences so Duo may require two "the".
In English you need the article "the" in this sentence . You would need to say "the last animals and the last trees". Or "the last animals and last trees". But you do need a "the" in the sentence. If you are not sure where to put it, it is safer to just use it ("the") before each in this instance.