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)

;)