Find, Findwithtag, findgameobjectwithtag Not working

Hi, my unity 2018.2.14f1 not finding game object, i dont know whats wrong with it, Code is below i have tried it with FindWithTag,FindGameObjectWithTag too, this prefab has name Ball and the tag too, whats wrong with it? can anyone please help me. if I make it public, and drop prefab manually, it works fine, but not finding it.

 public class Projectile : MonoBehaviour
 {
 
     public Transform target;
     public Transform throwPoint;
     private GameObject FireBall;
     public float timeTillHit = 1f;
     public Slider AngleSlider;
 
     void Start ()
     {
         FireBall = GameObject.Find ("Ball");
     }
 
     public void FireTheBall ()
     {
         float xdistance;
         xdistance = target.position.x - throwPoint.position.x;
 
         float ydistance;
         ydistance = target.position.y - throwPoint.position.y;
 
         float throwAngle; // in radian
         //OLD
         //throwAngle = Mathf.Atan ((ydistance + 4.905f) / xdistance);
         //UPDATED
         throwAngle = Mathf.Atan ((ydistance + 4.905f * (timeTillHit * timeTillHit)) / xdistance);
         //OLD
         //float totalVelo = xdistance / Mathf.Cos(throwAngle) ;
         //UPDATED
         float totalValue = xdistance / (Mathf.Cos (throwAngle) * timeTillHit);
 
         float xValue, yValue;
         xValue = totalValue * Mathf.Cos (throwAngle);
         yValue = totalValue * Mathf.Sin (throwAngle);
 
         if (FireBall == null) {
             FireBall = GameObject.Find ("Ball");
         }
         GameObject bulletInstance = Instantiate (FireBall, throwPoint.position, Quaternion.Euler (new Vector3 (0, 0, 0))) as GameObject;
         Rigidbody2D rb;
         rb = bulletInstance.GetComponent<Rigidbody2D> ();
 
         rb.velocity = new Vector2 (xValue, yValue);
     }
 
 
     public void MoveTarget ()
     {
         target.position = new Vector3 (0f, (float)AngleSlider.value);
     }
 }
 //END-Class