Did you try to place the backpack at the exact spot? There‘s a Tiny spot you can place backpack on. If thats not the problem could you describe the problem to me or atleast what you didnt get to work?
You have to put it back into the backpack by looking for a free space in backpack and pressing f or by tapping on a free spot in the backpack on mobile. If you drop it somewhere it dissapears from your backpack and you have to get it again.