How to make click event so when click button points get decreased?

So, I made when you get close to NPC and click "E" UI is showed and you can but things. My buttons have OnClickEvent "ButItem()". Anyone knows how to fix that? Points dont get decreased.

  GameManager gameManager;
     
     public int value;


 void Start()
         {
             gameManager = GameObject.FindGameObjectWithTag("GameManager").GetComponent<GameManager>();
         }
     
         // Update is called once per frame
         public void BuyItem()
         {
             gameManager.skullPoints -= value;
         }

GameManager script:

 public int skullPoints = 0;