My Healing DeveloperProduct Won't work, Whats the problem?

I don't know if this correct

-- setup local variables
local MarketplaceService = Game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
local productId = 19433950

-- 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 healing the player.
                player.Character.Humanoid.Health = 100
                -- more feedback for the player.
                game.Workspace.DisplayScreen.SurfaceGui.TextBox.Text = player.Name .. " has donate and get free full health."
            end
        end
    end 

    -- record the transaction in a Data Store
    local playerProductKey = "player_" .. receiptInfo.PlayerId .. "_purchase_" .. receiptInfo.PurchaseId
    ds:IncrementAsync(playerProductKey, 1)  

    -- tell ROBLOX that we have successfully handled the transaction
    return Enum.ProductPurchaseDecision.PurchaseGranted     
end

Then the LocalScript in StarterPack

local MarketplaceService = Game:GetService("MarketplaceService")
local buyButton = game.Workspace.BuyButton.SurfaceGui.TextButton
local productId = 19433950

buyButton.MouseButton1Click:connect(function()
    MarketplaceService:PromptProductPurchase(player, productId)
end)

My used models BuyButton, Display screen, Damage Health (i deleted that)

PLEASE HELP ME!