?General KeyDown() Function needs help :D

:FindFirstChild() accepts a string, not an object.

The Player variable is an object, because it is referring to game.Players.LocalPlayer.

So instead of this:

Human = game.Workspace:FindFirstChild(Player)

Syntactically, the correct way is this:

Human = game.Workspace:FindFirstChild(Player.Name)

I do not recommend you to access the player in this manner, because in some cases there could be names of objects that are the same as the Player's name in the Workspace.

Assuming that your script is a localscript, you can access the Character with simply:

game.Players.LocalPlayer.Character

Therefore, the best way to access the character model is this:

Human = Player.Character

Since a variable takes the place of something else, you do not need to write the whole name of the player model.

Also, I'd refer the player's mouse with a separate variable, so it would look cleaner.

local Player = game.Players.LocalPlayer
local Mouse = Player:GetMouse()

Mouse.KeyDown:connect(function(key)
    if string.lower(key) == "r" then
        local Human = Player.Character
        Human:WaitForChild("Humanoid").Health = 0
    end
end)

I would suggest using InputBegan of UserInputService instead of KeyDown.

Localscript:

local UserInput = Game:GetService("UserInputService")

UserInput.InputBegan:connect(function(Input) --Equivalent to keydown.
    if Input.KeyCode == Enum.Keyboard.R then
        Game.Players.LocalPlayer.Character:BreakJoints() --Killing the player by breaking joints.
    end
end)

local Player = game.Players.LocalPlayer:GetMouse()--This get's the mouse of your current LocalPlayer in players.
function KeyDown(key)--We create a parameter
if key:lower() == "r" then--We create any lower case key here we want to use.
Human = game.Players.LocalPlayer.Character.Humanoid---A variable to make it simple.
Human.Health = 0--Killing the player afterwards.
end
end
Player.KeyDown:connect(KeyDown)