ServerScriptService.Script:4: attempt to index field 'LocalPlayer' (a nil value)?

There are two sides to Roblox games. There are the clients, and there is the server. The server doesn't, and shouldn't, handle user input.

Think of this; when the server tried to find a LocalPlayer, which player does it pick? There may be multiple players connected to the game, and it can't just pick one at random.

Your script works in Studio because everything is ran on your computer locally, and there is no server. In an actual game, that's not the case.

The solution is to use a LocalScript, in StarterPack, or somewhere else LocalScripts can run. Then the Local Script will replicate inside the players when they join, and they'll run locally.

Code ran on the client normally doesn't replicate to the server if Network Filtering is enabled.

-- Local Script in StarterPack

--// Services
local Players = game:GetService("Players") -- About the same as game.Players

--// Local Variables
local plr = Players.LocalPlayer

Now, to get user input you should not use KeyDown. KeyDown is deprecated. Instead, I recommend UserInputService.

Here's an example of the finished code:

-- Local Script in StarterPack

--// Services
local Players = game:GetService("Players")
local UIS = game:GetService("UserInputService")

--// Local Variables
local plr = Players.LocalPlayer



-- Input Began
UIS.InputBegan:Connect(function(input)
	if input.KeyCode == Enum.KeyCode.Q then
		print("Q was pressed!")
	end
end)

-- Input Ended
UIS.InputEnded:Connect(function(input)
	if input.KeyCode == Enum.KeyCode.Q then
		print("Q was let go!")
	end
end)