Developer Product won't add 10 health when purchased?

a = player.Character.Humanoid.MaxHealth 

local MarketplaceService = Game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
local productId = 19697165 -- Put id here for it


MarketplaceService.ProcessReceipt = function(receiptInfo) 


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



            if receiptInfo.ProductId == productId then


                player.Character.Humanoid.MaxHealth = a + 10 -- Where the health is added 


                game.Workspace.DisplayScreen.SurfaceGui.TextBox.Text = player.Name .. " has purchased a healing Upgrade." -- Tells the player about the purchase
            end
        end
    end 


    local playerProductKey = "player_" .. receiptInfo.PlayerId .. "_purchase_" .. receiptInfo.PurchaseId
    ds:IncrementAsync(playerProductKey, 1)  


    return Enum.ProductPurchaseDecision.PurchaseGranted     
end

Have I done this completely wrong? Because it is not working and remember I am only a beginner. I wanted to make this into a GUI too if anyone has experience on that.

Thanks