Detecting a button click

I'm trying to spawn a unit in a 2D turn based war game, but the unfortunate reality is the On-click command doesn't work, it's included in the code below but it is completely dis-functional

does anyone know why i can;t use the On-click method here?

Code is included below with comments to highlight problem code

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 
 public class PlayerBehavior : MonoBehaviour {
     
     //public bool PlayersTurn;
     //public int ActionsPerTurn;
     public BoardManager boardManager;
     
     private Vector2 SelectedPosition;
     private GameObject Mbutton;
     private GameObject Bbutton;
     private GameObject Abutton;
     //private bool enemysTurn;
     
     void Start () {
         boardManager = GetComponent<BoardManager> ();
         Mbutton = GameObject.FindGameObjectWithTag("SpawnMsoldier");
         Abutton = GameObject.FindGameObjectWithTag("SpawnAsoldier");
         Bbutton = GameObject.FindGameObjectWithTag("SpawnBSoldier");
         Mbutton.gameObject.SetActive(false);
         Bbutton.gameObject.SetActive(false);
         Abutton.gameObject.SetActive(false);
     }
     // Update is called once per frame
     void Update () {
         
         if (Input.GetMouseButtonDown(0))
         {
             ClickSelect();
             Debug.Log(ClickSelect());
             if (ClickSelect() == GameObject.FindGameObjectWithTag("PlayerHQ"))
             {
                 Mbutton.gameObject.SetActive(true);
                 Bbutton.gameObject.SetActive(true);
                 Abutton.gameObject.SetActive(true);
                                 //This is the problem code, for some reason, onClick is not useable
                 if (Mbutton.onClick == true)
                 {
                     Debug.Log("Testing..");
                     Instantiate (boardManager.PlayerUnits[3],new Vector3 (1,1,0f),Quaternion.identity);        
                 }
                 
             }
             
             if (ClickSelect() != GameObject.FindGameObjectWithTag("PlayerHQ"))
             {
                 Mbutton.gameObject.SetActive(false);
                 Bbutton.gameObject.SetActive(false);
                 Abutton.gameObject.SetActive(false);
             }
             
         }
         
     }
     GameObject ClickSelect()
     {
         RaycastHit2D hit = Physics2D.Raycast(Camera.main.ScreenToWorldPoint(Input.mousePosition), Vector2.zero);
         
         if (hit)
         {
             Debug.Log(hit.transform.name);
             return hit.transform.gameObject;
         }
         else return null;
     }
 }