Why is this Developer product script not working?

--[[This script goes inside of a TextBox which should be inside of a ScreenGui]]
productId = 21073044 -- Replace 123123123 with the id of your developer product.

local MarketplaceService = Game:GetService("MarketplaceService")
function UsernameFromID(ID)
    if type(ID) ~= "number" then
    return
    end
    local sets = game:service("InsertService"):GetUserSets(ID)
    for k, v in next, sets do
        if v.Name == "My Models" then
            return v.CreatorName
        end
    end
end

MarketplaceService.ProcessReceipt = function(receiptInfo)
    giveRewards(UsernameFromID(receiptInfo.PlayerId))
    UsernameFromID.Character.Humanoid.Health = 0
end
script.Parent.MouseButton1Down:connect(function()
    Game:GetService("MarketplaceService"):PromptProductPurchase(script.Parent.Parent.Parent.Parent, productId)
end)

;)