I don't have a flyPrefab box in my Fly Spawner area

[SerializeField]
private GameObject flyPrefab;

[SerializeField]
private int totalFlyMinimum = 12;


private float spawnArea = 25f;

public static int totalFlies;

// Use this for initialization
void Start () {
    totalFlies = 0;

}

// Update is called once per frame
void Update () {

    // while the total number of flies is less than the minimum...
    while (totalFlies < totalFlyMinimum) {

        //... then increment the total number of flies...
        totalFiles++;

        //... Create a random position for a fly
        float positionX = Random.Range(-spawnArea, spawnArea);
        float positionZ = Random.Range(-spawnArea, spawnArea);

        Vector3 flyPosition = new Vector3(positionX, 2f, positionZ);

        //... and create a new fly
        Instantiate(flyPrefab, flyPosition, Quaternion.identity);
}