"ProcessReceipt Callback" not working?

I'm inserting some in-game purchases in my game. I created all the Developer Products correctly, and I inserted all the necessary local scripts to prompt the purchases, and they work without any problem. I got this script from the developer page of Roblox in order to record all the transactions. However, when I test the game and I buy something, it doesn't do anything. It says that the purchase has been successfully completed, but then nothing happens. I tried to add a print message in the script several times in different positions, and it never printed anything. All the ProductIDs are right, I checked them several times. I also thought that maybe the problem was the fact that I inserted some "wait()" in the different productFunctions, but when I tried to remove them it still didn't work. I hope that someone can solve this. Thanks in advance. Here the script:

local MarketplaceService = game:GetService("MarketplaceService")
local DataStoreService = game:GetService("DataStoreService")
local Players = game:GetService("Players")
local purchaseHistoryStore = DataStoreService:GetDataStore("PurchaseHistory")
 
local productFunctions = {}
productFunctions[988367837] = function(receipt,player)
	if player.Character and player.Character:FindFirstChild("Humanoid") then
		player.Character.Humanoid.WalkSpeed = player.Character.Humanoid.WalkSpeed *3
		wait(20)
		player.Character.Humanoid.WalkSpeed = player.Character.Humanoid.WalkSpeed /3
		return true
	end
end
productFunctions[988368778] = function(receipt,player)
	if player.Character and player.Character:FindFirstChild("Humanoid") then
		player.Character.Humanoid.JumpPower=player.Character.Humanoid.JumpPower *3
		wait(20)
		player.Character.Humanoid.JumpPower=player.Character.Humanoid.JumpPower /3
		return true
	end
end
productFunctions[988370741] = function(receipt,player)
	if player.Character and player.Character:FindFirstChild("Humanoid") then
		player.Character.Humanoid.MaxHealth = 999999
        player.Character.Humanoid.Health = 999999
        local ForceField = Instance.new("ForceField",player.Character)
		wait(20)
		player.Character.Humanoid.MaxHealth = 100
        player.Character.Humanoid.Health = 100
        ForceField:Destroy()
		return true
	end
end

local function processReceipt(receiptInfo)
 	
	local playerProductKey = receiptInfo.PlayerId .. "_" .. receiptInfo.PurchaseId
	local purchased = false
	local success, errorMessage = pcall(function()
		purchased = purchaseHistoryStore:GetAsync(playerProductKey)
	end)
	
	if success and purchased then
		return Enum.ProductPurchaseDecision.PurchaseGranted
	elseif not success then
		error("Data store error:" .. errorMessage)
	end
 
	local player = Players:GetPlayerByUserId(receiptInfo.PlayerId)
	if not player then
		
		return Enum.ProductPurchaseDecision.NotProcessedYet
	end
	
	local handler = productFunctions[receiptInfo.ProductId]
 
	local success, result = pcall(handler, receiptInfo, player)
	if not success or not result then
		warn("Error occurred while processing a product purchase")
		print("\nProductId:", receiptInfo.ProductId)
		print("\nPlayer:", player)
		return Enum.ProductPurchaseDecision.NotProcessedYet
	end
 
	local success, errorMessage = pcall(function()
		purchaseHistoryStore:SetAsync(playerProductKey, true)
	end)
	if not success then
		error("Cannot save purchase data: " .. errorMessage)
	end
 
	return Enum.ProductPurchaseDecision.PurchaseGranted
end
MarketplaceService.ProcessReceipt = processReceipt