Why won't my "key Press" function not work?

Its simple,

local UserInputService = game:GetService("UserInputService")
UserInputService.InputBegan:Connect(function(key)
if key.KeyCode == Enum.KeyCode.A then -- Replace 'A' with your key
--run what you want to happen
end
end)

(ina LocalScript)

mouse.KeyDown I have never used, I don't think its a function on roblox either.

I recommend using UIS, or User Input Service. It is very simple to use.

local userInputService = game:GetService("UserInputService")

userInputService.InputBegan:Connect(function(processed, input)
    if not processed then
    if input.KeyCode == Enum.KeyCode.L then
        -- run code here.
    end
end)

KeyCode is always the lowercase letter. Pressing shift + l will still run your code.

the not processed part tells the script that if the player is typing into a text box (chat for example) it won't run the code since they didn't want to execute the code.

KeyDown is the old way I am pretty sure.

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
local Sound = game.SoundService.TheBox



mouse.KeyDown:connect(function(key)
    if key:lower() == "l" or key:upper() == "L" then
        Sound:Play()
    end
end)

Here is the new way using userinputservice

local player = game.Players.LocalPlayer
local mouse = player:GetMouse()
local Sound = game.SoundService.TheBox
local userInputService = game:GetService("UserInputService")

userInputService.InputBegan:Connect(function(input, processed)
    if input.KeyCode == Enum.KeyCode.L then
        Sound:Play()
    end
end)

It also might be input = Enum.KeyCode.L . I forget but if it doesn’t work again try that.