How can I fix this error?

I wanna make a quiz that has a sign whether the user's answer is correct or not. I was modifying the script from unity but failed. The script is here:

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

public class GameController : MonoBehaviour {

 public Text questionDisplayText;
 public Text scoreDisplayText;
 public Text timeRemainingDisplayText;
 public SimpleObjectPool answerButtonObjectPool;
 public GameObject statusJawaban;
 public Transform answerButtonParent;
 public GameObject questionDisplay;
 public GameObject roundEndDisplay;

 private DataController dataController;
 private RoundData currentRoundData;
 private QuestionData[] questionPool;
 
 private bool isRoundActive;
 private float timeRemaining;
 private int questionIndex;
 private int playerScore;
 private List<GameObject> answerButtonGameObjects = new List<GameObject>();

 // Use this for initialization
 void Start()
 {
     dataController = FindObjectOfType<DataController>();
     currentRoundData = dataController.GetCurrentRoundData();
     questionPool = currentRoundData.questions;
     timeRemaining = currentRoundData.timeLimitInSeconds;
     UpdateTimeRemainingDisplay();

     playerScore = 0;
     questionIndex = 0;

     ShowQuestion();
     isRoundActive = true;

 }

 private void ShowQuestion()
 {
     RemoveAnswerButtons();
     QuestionData questionData = questionPool[questionIndex];
     questionDisplayText.text = questionData.questionText;

     for (int i = 0; i < questionData.answers.Length; i++)
     {
         GameObject answerButtonGameObject = answerButtonObjectPool.GetObject();
         answerButtonGameObjects.Add(answerButtonGameObject);
         answerButtonGameObject.transform.SetParent(answerButtonParent);

         AnswerButton answerButton = answerButtonGameObject.GetComponent<AnswerButton>();
         answerButton.Setup(questionData.answers[i]);
     }
 }

 private void RemoveAnswerButtons()
 {
     while (answerButtonGameObjects.Count > 0)
     {
         answerButtonObjectPool.ReturnObject(answerButtonGameObjects[0]);
         answerButtonGameObjects.RemoveAt(0);
     }
 }

 public void AnswerButtonClicked(bool isCorrect)
 {
     if (isCorrect)
     {
         playerScore += currentRoundData.pointsAddedForCorrectAnswer;
         scoreDisplayText.text = "Score: " + playerScore.ToString();
         GameObject.Find("Image ("+questionIndex+") ").GetComponent<Image>().color = Color.green;
         Debug.Log("Jawaban Benar");
     }

     if (questionPool.Length > questionIndex + 1)
     {
         questionIndex++;
         ShowQuestion();
         GameObject.Find("Image ("+questionIndex+")").GetComponent<Image>().color = Color.red;
         Debug.Log("Jawaban Salah");
     }
     else
     {
         EndRound();
     }

 }

 public void EndRound()
 {
     isRoundActive = false;

     questionDisplay.SetActive(false);
     roundEndDisplay.SetActive(true);
 }

 public void ReturnToMenu()
 {
     SceneManager.LoadScene("MenuScreen");
 }

 private void UpdateTimeRemainingDisplay()
 {
     timeRemainingDisplayText.text = "Time: " + Mathf.Round(timeRemaining).ToString();
 }

 // Update is called once per frame
 void Update()
 {
     if (isRoundActive)
     {
         timeRemaining -= Time.deltaTime;
         UpdateTimeRemainingDisplay();

         if (timeRemaining <= 0f)
         {
             EndRound();
         }

     }
 }

}