How do I fix this Dev. product? (Walkspeed)

local plr = game.Players.LocalPlayer
local link = game:GetService("MarketplaceService")
deb = 0


script.Parent.MouseButton1Click:connect(function()
local marketId = 21255472
link:PromptProductPurchase(plr,marketId)
link.ProcessReceipt = function(receiptInfo)
if Enum.ProductPurchaseDecision.PurchaseGranted and receiptInfo.PlayerId == plr.userId then
if deb == 0 then
deb = 1
plr.Character.Humanoid.WalkSpeed = 30
wait(1)
deb = 0
end
end
end
end)

This is for a GUI, when you click it it won't show the Purchase