Error in a really short script

This is only the part of the code that answer the question, but you can't create a c# file and paste just that. Even more so if it's a js file. Have a look at how c# script is done. Basically, you need a class inheriting from monobehaviour, put your methods inside, and have the "using UnityEngine" at the top of the file, outside the class.

you are mixing C# and javascript, in c# it looks like this:

 using UnityEngine;
 using System.Collections;
 
 public class PoolLib : MonoBehaviour {
     bool isPaused;
  
     void Start (){
     isPaused = false;
     }
      
     void OnGUI (){ 
         if (!isPaused){
             if (GUI.Button(new Rect(10,10,50,50), "Pause")){
                 Time.timeScale = 0f;
                 isPaused = true;
             }
         }
         if (isPaused){
             if (GUI.Button(new Rect(10,10,50,50), "Play")){
                 Time.timeScale = 1.0f;
                 isPaused = false;
             }
         }
     }
 }