custom menu lua

[ERROR] gamemodes/test/gamemode/custom_menu.lua:5: attempt to index global 'vgui' (a nil value)
  1. unknown - gamemodes/test/gamemode/custom_menu.lua:5
   2. unknown - lua/includes/modules/concommand.lua:54


local Menu

function gameMenu()
    if (Menu == nil) then
        Menu = vgui.Create( "DFrame" )
        Menu:SetSize(500,500)
        Menu:SetTitle("GameMode")
        Menu:SetDraggable(false)
        Menu:ShowCloseButton(false)
        Menu:SetPos(ScrW()/2 - 250, ScrH()/ 2 - 250)
        Menu:SetDeleteOnClose(false)
        Menu.Paint = function()
            surface.SetDrawColor(60,60,60,255)
            surface.DrawRect(0,0,Menu:GetWide(),Menu:GetTall() )

            surface.SetDrawColor(40,40,40,255)
            surface.DrawRect(0,24,Menu:GetWide(),1)
        end
    else
        if (Menu:IsVisible()) then
            Menu:SetVisible(false)
            gui.EnableScreenClicker(false)
        else
            Menu:SetVisible(true)
            gui.EnableScreenClicker(true)
        end
    end
end

concommand.Add("open_game_menu", gameMenu)