GameManager.cs

using System.Collections;
using TMPro;
using UnityEngine;

public class GameManager : MonoBehaviour
{
    public Animator UIanimator;
    public Rigidbody[] ballRBs;
    public TMP_Text timeScaleText;
    public TMP_Text countdownText;

    private bool uiOpen = false;
    private bool inCountdown = false;
    private int countdownSecond = 0;

    // Update is called once per frame

    private void Start()
    {
        countdownText.text = "";
    }

    private void Update()
    {
        if (Input.GetKeyDown(KeyCode.Escape))
        {
            if (!uiOpen && !inCountdown)
                PauseFunction();
            else if (uiOpen && !inCountdown)
                ResumeFunction();
        }

        if (Input.GetKeyDown(KeyCode.F))
        {
            if (!uiOpen && !inCountdown)
                AddForceToBalls();
        }

        timeScaleText.text = "TimeScale: "+Time.timeScale;

    }

    private void AddForceToBalls()
    {
        foreach (Rigidbody rb in ballRBs)
        {
            rb.AddForce(rb.transform.up * 100f *Random.Range(2,5));
        }
    }

    private void PauseFunction()
    {

        Time.timeScale = 0;
        uiOpen = true;
        UIanimator.SetBool("openUI", uiOpen);
    }

    private IEnumerator ResumeGame()
    {
        uiOpen = false;
        UIanimator.SetBool("openUI", uiOpen);
        inCountdown = true;
        countdownSecond = 3;

        while (countdownSecond > 0 )
        {
            countdownText.text = countdownSecond.ToString();
            countdownSecond--;
            yield return new WaitForSecondsRealtime(1f);
        }

        inCountdown = false;
        countdownText.text = "";
        Time.timeScale = 1;
    }

    public void ResumeFunction()
    {
        StartCoroutine(ResumeGame());
    }

}