Why am I getting this BCE0019 error? ScreenToWorldPoint' is not a member of 'System.Type

So, I'm very new to unity, and I'm trying to code a 2d game.

Below is the code I'm using right now, along with the error I'm getting.

 var mainCam = Camera;
 
 var topWall : BoxCollider2D;
 
 var bottomWall: BoxCollider2D;
 
 var Player01 : Transform;
 
 
 function Update () {
 
     topWall.size = new Vector2 (mainCam.ScreenToWorldPoint (new Vector3 (Screen.Width * 2f, 0f, 0f)).x, 1f);
     topWall.center = new Vector2 (0f, mainCam.ScreenToWorldPoint (new Vector3 (0f, Screen.height, 0f)).y + 0.5f);

 }

Assets/GameSetup.js(14,45): BCE0019: 'ScreenToWorldPoint' is not a member of 'System.Type'.

^ that is the error I'm getting.

I'm just going through a tutorial on youtube right now, and have triple checked that I have everything typed as he does. Any idea why I am getting this?