local scripts dont work (normally) in Workspace
make a Server Script and try :
local PP = script.Parent.ProximityPrompt
local RS = game.ReplicatedStorage
local item = RS:WaitForChild("DeliveryBox")
PP.Triggered:Connect(function(player)
local backpack = player.Backpack
local cloneItem = item:Clone()
cloneItem.Parent = backpack
cloneItem.Name = "Box"
end)