"Please put this paper on the fridge."
冷蔵庫の前に = in front of the fridge
冷蔵庫の正面に = on the front side of the fridge
冷蔵庫の中に = inside the fridge
冷蔵庫に = at the fridge (does not specify exactly the location, can be inside, outside. The paper needs to adhere to the fridge in some form: if it is in front of the fridge on a table, then it needs to be explicitly add の前に)
If you stick a paper, common sense is that it is on the fridge (outside) not inside. It can be inside the fridge, just that it is not common sense. Duolingo accepts answers from common sense.