What does this index value mean? It keeps erroring every time I test.

I'm very new to scripting with this kind of stuff, and I don't know what else to do for this event.

Basically, I have a Local Script which connects to a Server script. I want the Server script to execute commands to other scripts in the bus, but I ran the Script Debug errors and one error came up as "attempt to index local 'Bus' (a nil value)"

Here's the scripts:

Local script:

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)

Server script:

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)

If anyone can please help me out that would be amazing!

The problem here is exactly what the error says: Bus is nil.

When the FindFirstChild function cannot find the object it has been asked to look for, it returns nil. Since you set Bus to be a result of FindFirstChild, there is a possibility of it being nil.

Instead, you should use WaitForChild, which will yield the script until the child exists, then the script will continue.

Hope this helps! :)

EDIT:

-- instead of this
local Bus = game.Workspace:FindFirstChild(game.Players.LocalPlayer.PlayerGui.TGUI.Value)

-- do this
local player = game.Players.LocalPlayer
local playerGui = player:WaitForChild("PlayerGui")
local tGui = playerGui:WaitForChild("tGui")
local Bus = workspace:WaitForChild(tGui.Value)

-- or if your bus will always have the same name you can simply do
local Bus = workspace:WaitForChild("Bus")