How to send data when teleporting a player between games?

I'm making a level selection system and im trying to send the level number (single integer) to the other game through teleporting. I used the Roblox Dev website [https://create.roblox.com/docs/mechanics/teleporting-between-places]

but after following the instructions I get hit with an error code saying the data im sending through doesn't exist/is nil (attempt to index nil error)

Could someone help me find out what im doing wrong or if theres anything I need to add/change? Thank you very much.

Here is the code:

Server Script that sends info after pressing button

local teleportservice = game:GetService("TeleportService")
local teleportdata = {

    LevelNumber = 1
}

local teleportoptions = Instance.new("TeleportOptions")
teleportoptions:SetTeleportData(teleportdata)


local player = game:GetService("Players").PlayerAdded:Wait()
local placeid = 11338334502
local teleportgui = game.ReplicatedStorage.TeleportGui
teleportservice:SetTeleportGui(teleportgui)

script.Parent.MouseButton1Click:Connect(function()
    teleportservice:TeleportAsync(placeid, {player}, teleportoptions)
end)

Server Script that recives info

local Players = game:GetService("Players")

local function onPlayerAdded()
    local player = game:GetService("Players").PlayerAdded:Wait()
    local joinData = player:GetJoinData()
    local teleportdata = joinData.teleportdata
    local levelNumber = teleportdata.LevelNumber

    print(player.Name .. "joined with level " .. levelNumber)

end

Players.PlayerAdded:Connect(onPlayerAdded)