How shall I fix this?

player = script.Parent.Parent
 mouse = player:GetMouse()
 PartsLeft = game.Workspace.PartsLeft
function colorChange()
    if mouse.Target ~= nil then
    PartsLeft.Value = PartsLeft.Value + 1
    end
end

mouse.Button1Down:connect(colorChange)

I put this script in StarterGUI so that the player will be identified more easily. The thing is that when I play in "Solo Mode" it works just fine but when I go into "Start Server with 2 people" the output error says that Attempt to index global "mouse". What is wrong with this script? How come the players in the server can not do the same thing as in "Solo Mode". I also tried to put the script into Workspace, and identify player as game.Players:getChildren() but it did not work.