public Camera cam; not working with Unity [duplicate]

Code that generates an error:

void Update()
{
    if (Input.touchCount > 0)
    {
        RaycastHit2D hit = Physics2D.Raycast(Camera.main.ScreenToWorldPoint(Input.GetTouch(0).position), Vector2.zero);
        if (hit && hit.collider != null && hit.collider.name == "leftTapArea")
        {
            hit.transform.name = "Hit";
        }
    }
}

It says that something is wrong with this string:

RaycastHit2D hit = Physics2D.Raycast(Camera.main.ScreenToWorldPoint(Input.GetTouch(0).position), Vector2.zero);

Error:

NullReferenceException: Object reference not set to an instance of an object leftScript.Update () (at Assets/leftScript.cs:16)

The only thing that can return null in your code is Camera.main.ScreenToWorldPoint. It means that Camera.main is null. For Camera.main to be initialized, the camera must have the MainCamera tag.

Select the Camera GameObject then change the tag to MainCamera.

If you don't want your camera to be in the MainCamera tag, you can also find wit directly with GameObject.Find then get the Camera component from it.

Camera cam;

void Start()
{
    cam = GameObject.Find("NameOfCameraGameObject").GetComponent<Camera>();
}

void Update()
{
    if (Input.touchCount > 0)
    {
        RaycastHit2D hit = Physics2D.Raycast(cam.ScreenToWorldPoint(Input.GetTouch(0).position), Vector2.zero);
        if (hit && hit.collider != null && hit.collider.name == "leftTapArea")
        {
            hit.transform.name = "Hit";
        }
    }
}

Make sure you have in your scene an active gameobject with the Camera component and the tag "MainCamera"

_{(source: unity3d.com)}