Why does not the button animatIon in Unity?

Question

After creating the GameObject in the Hierarchy, I added a Button component.

And I set the Transtion setting to Animation.

And we created an animation by adding Sprite image for Normal, Highlited, Pressed, and Disabled functions.

Below is a picture of the setting.

I have succeeded in finding the GameObject by applying the Find function to the script.

The script looks like this:

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;

using wp.events;

public class ButtonController : MonoBehaviour, IWordGame
{
    private Button start_btn, play_btn;

    void Awake()
    {

    }

    void Start ()
    {
        WordGameEventManager.Instance.addEvent(onEventHandler); // 이벤트 추가

        start_btn = GameObject.Find("start_btn").GetComponent<Button>();
        start_btn.onClick.AddListener(()=> onClickStartBtn(start_btn));

        play_btn = GameObject.Find("btn_play").GetComponent<Button>();
        Debug.Log("play_btn Find >> " + play_btn);
        play_btn.onClick.AddListener(() => onClickPlayBtn(play_btn));
    }

    void Update ()
    {

    }

    public void onEventHandler(object obj, EventArgs e)
    {
        WordGameEvent wg_evt = e as WordGameEvent;
        //Debug.Log("ButtonController was Receive Event >> " + wg_evt.eventType);
    } 

    public void onClickStartBtn(Button sbtn)
    {
        //Debug.Log("You Click Start_Button");
        WordGameEventManager.Instance.dispatchEvent(sbtn, EventTypes.PUSH_START_BTN); // 이벤트 보냄
    }

    public void onClickPlayBtn(Button pbtn)
    {
        Debug.Log("You Click Play_Button");
        Hashtable data = new Hashtable();

        data.Add("word", WordGameManager.Instance.getQuizQuestion());
        Debug.Log("data value >> " + data);

        WordGameEventManager.Instance.dispatchEvent(pbtn, EventTypes.PUSH_WORD_BTN, data);
    }
}

If I click the button, the Pressed Animation does not occur and the onClickPlayBtn function does not execute.

What is wrong and not running? I would really appreciate it if you would advise.

Note that the script above is currently in MainCamera.