I fixed it myself. Here is the fixed script:
game.Players.PlayerAdded:connect(function(plr)
if game.Players.NumPlayers >= 2 then
msg =Instance.new("Hint", game.Workspace)
msg.Text = "Game Start!"
else
msg = Instance.new("Hint", game.Workspace)
msg.Text = "2 players needed to start."
end
end)

2 comments
I fixed it before I saw your answer. But ill still give you the point because its true!
Heh, I was typing my answer as you updated your's. Sorry.