The title says it all. I am using this in a localscript in StarterGui. I believe this is how you are supposed to use :SetCoreGuiEnabled. So I went to the API and I believe I have it set up correctly.
game.StarterGui:SetCoreGuiEnabled(Enum.CoreGuiType.Backpack, false)
game.StarterGui:SetCoreGuiEnabled(Enum.ControlMode.Classic, true)
And I get this error when I attempt to run it.
20:06:52.862 - Unable to cast token to token
20:06:52.863 - Script 'Players.Player.PlayerGui.LocalScript', Line 2
Any help is appreciated :)

2 comments
Well, I feel like the biggest idiot ever...
http://www.quora.com/Who-is-the-biggest-idiot-in-the-world i beg to differ