Help with updating lobby list (like in COD lobbies)?

This script is meant to update the player list ( much like in COD lobbies ) when a new player joins. However it says that ScreenGui is not a child of PlayerGui despite it definitely being there. Any idea whats going on?

game.Players.PlayerAdded:Connect(function(plr)
	plr:WaitForChild("PlayerGui"):WaitForChild("ScreenGui")
	for i,v in pairs(game.Players:GetChildren()) do
		local ptclone = game.ServerStorage.PlayerTemplate:Clone()
		ptclone.Parent = v.PlayerGui.ScreenGui.MainMenu.Multiplayer.Players
		ptclone.Name = plr.Name
		ptclone.PlayerName.Text = plr.Name
	end
end)

When I use wait(5) ( to see if its just acting like this because the player isn't fully loaded in )

17:57:18.348 - ScreenGui is not a valid member of PlayerGui

When I use :WaitForChild("ScreenGui")

17:55:56.771 - Infinite yield possible on 'Players.Player2.PlayerGui:WaitForChild("ScreenGui")'

NOTE: This works in quick test play, but doesn't work when I use the test tab to start a server.