How would one make an animation play in the 2D engine with the press of a button?

Hey guys. I'm working in the 2D unity engine. I'm trying to animate a character in the game, where the walk animation plays when the player presses a button. I have followed many tutorials for animating with similar problems, but I have a problem with my scripting. My current script is as follows:

using UnityEngine;

using System.Collections;

public class walk_animation : MonoBehaviour {

 void Update()
 {
     if (Input.GetKeyDown(KeyCode.W))
     {
         animation.Play("Walking", PlayMode.StopAll);
     }
 }

}

There is an error on the "animation.Play" command. Does anyone know a solution?