...
private int _score;

void Awake() {
   Messenger.AddListener(GameEvent.ENEMY_HIT, OnEnemyHit); 
}

void OnDestroy() {
   Messenger.RemoveListener(GameEvent.ENEMY_HIT, OnEnemyHit); 
}

void Start() {
   _score = 0;
   scoreLabel.text = _score.ToString();  
   
   settingsPopup.Close();
}

private void OnEnemyHit() {
   _score += 1;                      
   scoreLabel.text = _score.ToString();
}
...
