Player won't carry out animation?

You need to use UserInputService

you will need to use events of it:

> input began

> input ended

local UIS = game:GetService("UserInputService")
local Players = game:GetService("Players")

local Client = Players.LocalPlayer

local Animation = Instance.new("Animation")
Animation.AnimationId = "rbxassetid://6317159485"
Animation.Parent = Client.Character
Animation.Looped = true

local Current = nil

UIS.InputBegan:Connect(function(Self, GPE)
   if Self.KeyCode == Enum.KeyCode.E and not GPE then 
      Current = Client.Character:WaitForChild("Humanoid"):LoadAnimation(Animation)
      Current:Play()
   end
end)

UIS.InputEnded:Connect(function(Self, GPE)
   if Self.KeyCode == Enum.KeyCode.E and not GPE then 
      Current:Stop()
   end
end)

Tell me if this works, also:

:GetMouse() is incredibly deprecated, I don't recommend using it and .KeyDown detects a key when it's pushed and not when held.