How do I use TeleportPartyAsync or TeleportToPrivateServer?

I'm trying to create a script that teleports a group of players if they are "Ready" for the game, but whenever I try to use Teleport Party A Sync or Teleport To Private Server, it comes up with an HTTP 403 Error. Here's the code:

local timeleft = game:GetService("ReplicatedStorage"):WaitForChild("TimeLeft")
local teleserv = game:GetService("TeleportService")
local gameid = 3171170063

while true do
	for a = 1,timeleft.Value do
		timeleft.Value = timeleft.Value - 1
		wait(1)
	end
	for _,plr in pairs(game.Players:GetChildren()) do
		if plr:WaitForChild("Ready").Value == true and plr:IsA("Player") then
			print(plr.Name.." is ready to be teleported!")
			local reserve = teleserv:ReserveServer(gameid)
			teleserv:TeleportToPrivateServer(gameid,reserve,plr)
		else
			print(plr.Name.." was not ready for the party.")
		end
	end
	wait(5)
	timeleft.Value = 45
end