OnMouseEnter not working

The object using OnMouseEnter must have a collider. Check if yours does.

OnMouseEnter may not work if an other collider is occluding it. If this is the case, either prevent the other collider from occluding (maybe it can be disabled?), or do a manual check to see if the cursor is inside any object behind it:

 bool hovered = false;
         
 Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
 foreach(RaycastHit hit in Physics.RaycastAll (ray))
 {
     if (hit.collider == collider)
     {
         hovered = true;
         break;
     }
 }

where u attached script (where u make function OnMouseEnter )on cube or not, if u attached on cube then it will work perfectly.

if you don't want to use any ray casting, you can do this :

 var clicked = false;

 function OnMouseDown(){
 //click object multiple times to turn on or off. 
 //if mouse is being clicked else where, then noting should happen.
 clicked = !clicked;
 }
 
 function Update(){
 
     if (clicked == true){
 
     //what will happen when this is true
     }
 }

just keep in mind-

1.The object needs a collider

2.This script needs to go on the object you want to be clicking

 using UnityEngine;
 
 public class Interactable : MonoBehaviour 
 {
     [SerializeField] private Color _highlightedColor;
     private Color _originalColor;
     private Material _material;
 
     void Start() 
     {
         Renderer rend = GetComponent<Renderer>();
         _material = new Material (rend.material);
         rend.material = _material;
         _originalColor = _material.color;
     }
 
     void OnMouseEnter() 
     {
         _material.color = _highlightedColor;
     }
 
     void OnMouseExit()
     {
         _material.color = _originalColor;
     }
 }

You better create your own copy of the material so when you change it, other objects aren't affected.