My UI Score Is Not Working?

First, assign a gameobject with a Text component on it to the scoreText variable (drag it from hierarchy to the inspector, in Unity), it should work

I think you should avoid using PlayerPrefs in Update / FixedUpdate/OnGUI or everything that is executed every frame

In scoreUI, you get the "yourScore" value from playerpref every Update (every frame) You should use the currentScore variable in scoreKeeper.js Same in OnGUI , replace the PlayerPrefs.GetInt(...) by highScore.

For the countdown you can add a float variable somewhere, let say scoreUI.cs (btw, why do you use both cs and js ?), set it to 60 in the Start function and in update, add

 countDown -= Time.deltaTime;

deltaTime is the time in second between the last frame and the current one. Then make a timeText variable which will contains the text that display the time , and update it in Update It should looks like this

 public class scoreUI : MonoBehaviour
 {
 
     public Text scoreText, timeText;
     public float timeLeft;
 
     void Start()
     {
         timeLeft = 0;
         if (scoreText || timeText)
             Debug.LogError("Assign Text gameobjects in the inspector");
     }
 
     void Update()
     {
         scoreText.text = "Score: " + scoreKeeper.currentScore;
         timeText.text = "Time: " + timeLeft;
         timeLeft -= Time.deltaTime;
     }
 }