Problem with ScreenToWorldPoint and touches (C#)

ScreenToWorldPoint must receive a Vector3 where x and y are the screen coordinates, and z is the distance from the camera. Since you're passing a Vector2, the compiler promotes it to Vector3 setting the z coordinate to 0, what will always return the camera position, no matter what x and y are.

If must copy touch.position to a Vector3 variable and set its z component to the distance you want:

    Vector3 tch = touch.position;
    tch.z = 15; // define distance from the camera
    Vector3 vec = Camera.main.ScreenToWorldPoint(tch);
    Instantiate(....);

Unfortunately, this will create all objects at a fixed distance from the camera - or at the surface of a sphere with radius 15, in this case.
If you want to create the objects in a plane, you should use ScreenPointToRay instead: create the base plane, create the ray, use plane.Raycast to find the distance, and ray.GetPoint(distance) to find the point... a piece of cake, for sure! But don't panic, the whole thing is easier than it may look:

public class CreateObject : MonoBehaviour {

 public GameObject createdObject;
 // create a plane in xy, at z = 5
 Plane basePlane = new Plane(-Vector3.forward, new Vector3(0, 0, 5));

 void Update (){
    foreach (Touch touch in Input.touches){
      if (touch.phase == TouchPhase.Began){ 
        // no need to set z, because ScreenPointToRay ignores it
        Ray ray = Camera.main.ScreenPointToRay(touch.position);
        float distance;
        if (basePlane.Raycast(ray, distance)){
          Vector3 vec = ray.GetPoint(distance); // get the plane point hit
          Instantiate(createdObject, vec, Quaternion.identity);
        }
      }
    }
 }

}