"Dictionaries are not thin."
I'm a little confused by the difference between
じしょは うすく ありません。
The app requires the second sentence as translation.
Could it be that the second sentence is more helpfully translated as "There are no thin dictionaries" (because there are so many words that need to be explained)? Whereas one might use the first sentence to refer to a particular set of dictionaries, maybe adding この?
The second one is the "official" correct way to say it, the first one is the one that is actually used. They both mean exactly the same.