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.