Camera ScreenToWorldPoint Instatiating

I need a little help on a problem i have. i want to place an object in 2d space at my mouse position, so i used ScreenToWorldPoint. but it comes up as an error

 mousePos = Input.mousePosition;   //this is already defined at the start
             Vector3 placement = new Vector3 (Camera.main.ScreenToWorldPoint(mousePos));
 
         for(int i = 0; i < towers.Count; i ++)
         {
             if(towers[i].selected == true && Input.GetButtonDown ("Place"))
             {
                 towers[i].selected = false;
                 GameObject clone = Instantiate(towers[i].towerObject, placement, Quaternion.identity) as GameObject;
             }
         }

it says

(33,92): error CS1729: The type UnityEngine.Vector3' does not contain a constructor that takes 1' arguments

and

Object reference not set to an instance of an object