Unity 2D: Object moves only in Z axis.

I want to move an object by mouse. The object moves only in Z axis. It doesn't move in X and Y, they don't change. I need it to move in X and Y too.

I did next.
1. Added a cube.
2. Added the next script to the cube:

Code (CSharp):

1.     void Start () {
2.  
3.     }
4.  
5.     // Update is called once per frame
6.     public Vector3 targetPosition;
7.  
8.     // Update is called once per frame
9.     void Update()
10.     {
11.         if (Input.GetKeyDown(KeyCode.Mouse0))
12.         {
13.             targetPosition = Camera.main.ScreenToWorldPoint(Input.mousePosition);
14.             targetPosition.z += 10;
15.         }
16.  
17.         transform.position = Vector3.MoveTowards(transform.position, targetPosition, Time.deltaTime * 5);
18.     }

3. Switched to 2D.
4. Run the game.

 

The problem was when I created a 3D project and then switched to 2D. It didn't work.
But when I created a new 2D project, it worked.