Why wont this dev product work?

I am trying to make it so that once the player buys this product he will be rewarded with 50 credits. For some reason its not working.

Cash = script.Parent.Parent.Parent.Parent.Parent:WaitForChild("leaderstats"):WaitForChild("Credits")
-- setup local variables
local MarketplaceService = Game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
local productId = 20768659

-- define function that will be called when purchase finished
MarketplaceService.ProcessReceipt = function(receiptInfo) 

    -- find the player based on the PlayerId in receiptInfo
    for i, player in ipairs(game.Players:GetChildren()) do
        if player.userId == receiptInfo.PlayerId then

            -- check which product was purchased
            if receiptInfo.ProductId == productId then
                --handle purchase. In this case we are highering the player's WalkSpeed         
                function onClick()
                    if Cash.Value >= 0 then
                        Cash.Value = Cash.Value + 100
                -- more feedback for the player.
                game.Workspace.DisplayScreen.SurfaceGui.TextBox.Text = player.Name .. " has purchased a healing potion."
                    end
                end
            end
        end
    end 
end