Open Source Run System

This is a running system using the service known as "UserInputService". UserInputService, (abbreviated as UIS), practically detects input from the client.

local UIS = game:GetService("UserInputService")
local Player = game.Players.LocalPlayer
local Character = Player.Character or Player.CharacterAdded:wait()
local CanRun = Player:WaitForChild("CanRun")


UIS.InputBegan:Connect(function(input)
	   if input.KeyCode == Enum.KeyCode.LeftShift then
		if CanRun.Value == true then
       	 	local Anim = Instance.new("Animation")
        	Anim.AnimationId = "rbxassetid://5431913931"
        	PlayAnim = Character.Humanoid:LoadAnimation(Anim)
			PlayAnim:Play()
		end
    end
end)


UIS.InputEnded:Connect(function(input)
	    if input.KeyCode == Enum.KeyCode.LeftShift then
		if CanRun.Value == true then
			CanRun.Value = false
			 Character.Humanoid.WalkSpeed = 16
			 PlayAnim:Stop()
			 local Humanoid = Character:WaitForChild("Humanoid")
			 wait(1)
			 CanRun.Value = true
		end
    end
end)

Thats the full script. All functional, and etc.

Just so your aware, you need to use a script where it inserts a boolvalue into the player.

Make a script in ServerScriptService and put this code into it:

local CanRun = Instance.new("BoolValue",game.Players.LocalPlayer)
CanRun.Name = "CanRun"
CanRun.Value = true

That makes the script work, or else there would be multiple errors in the output bar as the system couldnt find the value CanRun inside of the player object.