Detecting Collisions with Sphere Collider

Hey Guys, I am trying to detect a collision with a sphere collider in my game, but am having a difficult time finding information on it. What I have is : A cylinder with an added "Sphere Collider" Component (added through the gui "Add component"). A block called "Unit" which is going to be colliding with the sphere collider. I am trying to make the sphere collider attached to the cylinder print a message when the block enters the collider field. I looked at the documentation and dont know what to do with the code:

 public class Tower : MonoBehaviour
 {
     // Update is called once per frame
         void Update ()
         {
         }
 
         void OnCollisionEnter(Collision col) {
             if(col.gameObject.name=="Unit"){
             Debug.Log(col.gameObject.name);
             }
         }
     }


 Anything would help guys. Thanks,