How to correctly use Developer Products?

I've only had one problem with DP before, but other than that, they always worked fine. Now, I tried accessing it through a GUI, and that's where I go wrong. Here's the script:

script.Parent.MouseButton1Click:connect(function()
	p = script.Parent.Parent.Parent.Parent.Parent
	script.Parent.Parent.Visible = false 
	if p.Order.Value == true then
		id = 20869844
game:GetService("MarketplaceService"):PromptProductPurchase(p, id)
	print('passed.5')
	wait()
	print('past 1st')
		print('past 2nd')
		-- setup local variables
local MarketplaceService = game:GetService("MarketplaceService")
wait()
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
MarketplaceService.ProcessReceipt = function(receiptInfo) 
		if p.userId == receiptInfo.PlayerId then -- Most likely this is the  line where it goes wrong.
			print('3') -- Never gets up to here
			if receiptInfo.ProductId == id then
				print('close')
				p.Order.Value = false
				print('dd')
		for i,v in pairs(p.Order:GetChildren()) do
			if v:IsA("BoolValue") then
			game.Lighting[v.Name]:clone().Parent = p.Backpack
			end
		end
		p.Order:ClearAllChildren()
				-- more feedback for the player.
		end
	end	
	
	-- record the transaction in a Data Store
	local playerProductKey = receiptInfo.PlayerId .. "_got_" .. receiptInfo.PurchaseId
	ds:IncrementAsync(playerProductKey, 1)	
	
	-- tell ROBLOX that we have successfully handled the transaction
	return Enum.ProductPurchaseDecision.PurchaseGranted		
end

	end
	script.Parent.Parent.Parent:Destroy()
end)

I've used this same template for any DP I ever had and it worked fine. This is a Script in a TextButton that is in a ScreenGui located in the PlayerGui.