Bad argument at line 4 of server script? New to RemoteEvents & Local Scripts.

I have 2 scripts which used to be 1 local script but now it is 2 scripts. What I want it to do is when a guy presses a key while he sits on the driver's seat of a bus, it will tell other scripts to do their function, for example opening the door.

I keep getting an error saying "bad argument #1 to 'lower' (string expected, got Object) at Line 4 of the Server Script.

Here are the scripts:

Local script:

local mouse = game.Players.LocalPlayer:GetMouse()
local event = game.ReplicatedStorage.BusEvent
local Bus = game.Workspace:findFirstChild(game.Players.LocalPlayer.PlayerGui.TGUI.Value.Value)

event:FireServer()

Server script:

local event = game.ReplicatedStorage.BusEvent

event.OnServerEvent:Connect(function(key)
key = string.lower(key)
if key == "q" then -- LEFT SIGNAL
Bus.Model.Bodykit.Blinkers.DriversCab.LeftEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.LeftEnable.Value
Bus.Model.Bodykit.Blinkers.DriversCab.RightEnable.Value = false
Bus.Model.Bodykit.Blinkers.DriversCab.HazardEnable.Value = false
elseif key == "e" then -- RIGHT SIGNAL
Bus.Model.Bodykit.Blinkers.DriversCab.RightEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.RightEnable.Value
Bus.Model.Bodykit.Blinkers.DriversCab.LeftEnable.Value = false
Bus.Model.Bodykit.Blinkers.DriversCab.HazardEnable.Value = false
elseif key == "x" then -- HAZARDS
Bus.Model.Bodykit.Blinkers.DriversCab.HazardEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.HazardEnable.Value
Bus.Model.Bodykit.Blinkers.DriversCab.LeftEnable.Value = false
Bus.Model.Bodykit.Blinkers.DriversCab.RightEnable.Value = false
elseif key == "k" then -- KNEEL
Bus.Model.Bodykit.Blinkers.DriversCab.KneelEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.KneelEnable.Value
elseif key == "m" then -- FRONT DOOR OPEN/CLOSE
Bus.Model.Bodykit.Blinkers.DriversCab.FrontEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.FrontEnable.Value
elseif key == "n" then -- BACK DOOR UNLOCK
Bus.Model.Bodykit.Blinkers.DriversCab.RearEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.RearEnable.Value
elseif key == "l" then -- HEADLIGHTS
Bus.Model.Bodykit.Blinkers.DriversCab.HeadlightsEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.HeadlightsEnable.Value
elseif key == "j" then -- INTERIOR LIGHTS
Bus.Model.Bodykit.Blinkers.DriversCab.InteriorEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.InteriorEnable.Value
elseif key == "r" then -- REVERSE
Bus.Model.Bodykit.Blinkers.DriversCab.ReverseEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.ReverseEnable.Value

end
end)

If you know what the problem is can you please help me solve the issue! I know nothing about scripting, so please help me out!

The first argument that OnServerEvent takes is always the player who fired the event. If you want to get the user's keyboard input, I suggest you use UserInputService.

Here's my take on this:

LocalScript:

local uiService = game:GetService("UserInputService")
local event = game.ReplicatedStorage.BusEvent
local mouse = game.Players.LocalPlayer:GetMouse()
local Bus = game.Workspace:FindFirstChild(game.Players.LocalPlayer.PlayerGui.TGUI.Value)

uiService.InputBegan:Connect(function(key)
	event:FireServer(key.KeyCode)
end)

ServerScript:

local event = game.ReplicatedStorage.BusEvent

event.OnServerEvent:Connect(function(player, key)
	local Bus = workspace:FindFirstChild(player.PlayerGui.TGUI.Value)
	if key == Enum.KeyCode.Q then -- LEFT SIGNAL
	Bus.Model.Bodykit.Blinkers.DriversCab.LeftEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.LeftEnable.Value
	Bus.Model.Bodykit.Blinkers.DriversCab.RightEnable.Value = false

	Bus.Model.Bodykit.Blinkers.DriversCab.HazardEnable.Value = false

	elseif key == Enum.KeyCode.E then -- RIGHT SIGNAL

	Bus.Model.Bodykit.Blinkers.DriversCab.RightEnable.Value = not 		Bus.Model.Bodykit.Blinkers.DriversCab.RightEnable.Value

	Bus.Model.Bodykit.Blinkers.DriversCab.LeftEnable.Value = false
Bus.Model.Bodykit.Blinkers.DriversCab.HazardEnable.Value = false
	elseif key == Enum.KeyCode.X then -- HAZARDS

	Bus.Model.Bodykit.Blinkers.DriversCab.HazardEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.HazardEnable.Value
	Bus.Model.Bodykit.Blinkers.DriversCab.LeftEnable.Value = false

	Bus.Model.Bodykit.Blinkers.DriversCab.RightEnable.Value = false
	elseif key == Enum.KeyCode.K then -- KNEEL

	Bus.Model.Bodykit.Blinkers.DriversCab.KneelEnable.Value = not 	Bus.Model.Bodykit.Blinkers.DriversCab.KneelEnable.Value
	elseif key == Enum.KeyCode.M then -- FRONT DOOR OPEN/CLOSE

	Bus.Model.Bodykit.Blinkers.DriversCab.FrontEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.FrontEnable.Value

	elseif key == Enum.KeyCode.N then -- BACK DOOR UNLOCK

	Bus.Model.Bodykit.Blinkers.DriversCab.RearEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.RearEnable.Value

	elseif key == Enum.KeyCode.L then -- HEADLIGHTS
	Bus.Model.Bodykit.Blinkers.DriversCab.HeadlightsEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.HeadlightsEnable.Value

	elseif key == Enum.KeyCode.J then -- INTERIOR LIGHTS

	Bus.Model.Bodykit.Blinkers.DriversCab.InteriorEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.InteriorEnable.Value

	elseif key == Enum.KeyCode.R then -- REVERSE

	Bus.Model.Bodykit.Blinkers.DriversCab.ReverseEnable.Value = not Bus.Model.Bodykit.Blinkers.DriversCab.ReverseEnable.Value

	end
end)

Not my most efficient code. Untested.

The UserInputService gets the player's input and then returns an InputObject. We use the KeyCode property to figure out which key they're using and then fire the RemoteEvent with the key's KeyCode as its first argument. Then, the OnServerEvent connects with the player who fired the event as its first argument and the KeyCode as its second argument.

We then see if a KeyCode matches the KeyCode we passed to the event. If it does, then you do whatever.