Why is this Dev Product Not Selling?

The following script below is the handler for my developer product. Everything seems to work, but it says in the output that User id is not a valid member of folder. Does anyone have any ideas?

local MarketPlaceService = game:GetService('MarketplaceService')
local devProductID = 137200600 --1000 cash

MarketPlaceService.ProcessReceipt = function(receiptInfo)
    for i, player in ipairs(game.Players:GetChildren())do
        if player.userId == receiptInfo.PlayerId then
            if receiptInfo.ProductId == devProductID then 
                player.leaderstats.Cash.Value = player.leaderstats.Cash.Value + 1000
            end
        end
    end
    return Enum.ProductPurchaseDecision.PurchaseGranted
end