This gamepass fails to confirm and I don't know why?

Whenever a player tries to buy the credits, the buying gui or the prompt(whatever you wanna call it) it gets stuck on the loading screen and then nothing happens. But in testing mode, it works well. I think this script is the problem.

local MarketplaceService = game:GetService('MarketplaceService')
local devproduct = 52846482 --100 Credits

MarketplaceService.ProcessReceipt = function(receiptInfo)
 for i, player in ipairs(game.Players:GetChildren()) do
  if player.userId == receiptInfo.PlayerId then
   if receiptInfo.ProductId == devproduct then
   player.Credits.Value = player.leaderstats.Cash.Value + 100
   end
  end
 end

 return Enum.ProductPurchaseDecision.PurchaseGranted
end

If I check the output in game it says PurchasePromptScript: onAcceptPurchase() failed because HTTP 0(HTTP 403 (HTTP/1.1 403 Forbidden))