I want to activate a game object when pressing a button but when i click play,i can press anywhere on the screen to activate it

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

public class RunAnim : MonoBehaviour {

 public GameObject modify;

 void Update()
 {
      if (Input.GetMouseButton(0))
         modify.SetActive (true);
     else
         modify.SetActive (false);
 }

}

Input.GetMouseButton is called whenever the mouse is clicked, if you want your button to be part of the UI (i.e. a 2D object which stays in from of the camera) use the UI button like in this tutorial, or if you want a 3D object to be clickable you’ll want to use raycastin, like here.