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