"The toy is between the chair and the desk."
Translation:장난감이 의자하고 책상 사이에 있어요.
Ahh I see. If they had used the 의 for 사이에 in another sentence but not here, they should comment on whether or not it's required at all. In my experience people just skip it and say "(thing)하고 (thing) 사이에." It's a con of having just the app as it's more difficult for the creators to reach out to those taking the course.
It is omitted a lot. Door of the room, color of the house, dog's bone, kid's toy and many oher phrases would would usually be spoken without the "의" particle, but "almost always" might be an overstatement. It's contracted into 네 and 내 for yours and mine. It's used for for 3rd person possession to avoid confusion with simple "그" And I think there a number of sentences or phrases that would sound strange or possibly be misinterpreted without it. 돈에 대한 애착이 모든 악의 근원[뿌리]이 된다.