My animation will not stop once i press the keybind how can i fix this?

The reason its not stopping is because in the end you're creating a new one then stopping it so this is how to fix.

local Player = game.Players.LocalPlayer
local Character = Player.Character or script.Parent
local Humanoid = Character.Humanoid
local UserInputService = game:GetService("UserInputService")
local AnimationId = 'rbxassetid://4397242217'
local Key = 'W'

UserInputService.InputBegan:Connect(function(Input, ItTyping)
    if ItTyping then return end
    if Input.KeyCode == Enum.KeyCode[Key] then
        local Animation = Instance.new("Animation")
        Animation.AnimationId = AnimationId
        local LoadAnimation = Humanoid:LoadAnimation(Animation)
        LoadAnimation:Play()
	Animation.Parent = Character
	Animation.name = "Animation"
    end
end)

UserInputService.InputEnded:Connect(function(Input, IsTyping)
    if IsTyping then return end
        if Input.KeyCode == Enum.KeyCode[Key] then
	    if Character:FindFirstChild("Animation") == true then
            Humanoid:LoadAnimation(Character:FindFirstChild("Animation")):Stop()
        end
    end
end)