Why does it give me an error saying: attempt to index nil with 'GetMouse'?

The code is inside a script of a VehicleSeat

local seat = script.Parent
local LightPart = script.Parent.Parent.LightParts
local on = false

seat.Changed:Connect(function(ch)
	if ch == "Occupant" then
		if seat.Occupant then
			local player = game.Players:FindFirstChild(seat.Occupant.Parent.Name) --The occupant is the humanoid, the humanoid.Parent is the player's character

			--Your code here
			game.Players.LocalPlayer:GetMouse().KeyDown:Connect(function(key)
				if key == "l" then
					if on == true then
						on = false
						LightPart.HeadLightPart.Material = Enum.Material.SmoothPlastic
						LightPart.HeadLightPart2.Material = Enum.Material.SmoothPlastic
					else
						LightPart.HeadLightPart.Material = Enum.Material.Neon
						LightPart.HeadLightPart2.Material = Enum.Material.Neon
					end
				end
			end)
		end
	end
end)

game.Players.LocalPlayer.Character.Humanoid.Seated:Connect(function(val, seat)

	--val is true or false, based on the player is sitting or not
	--seat is the Seat part that player sat on

end)

You should use this code (official article). The seat is an Instance, so you can specify the vehicle through that.