Error in Camera Tutorial that I can't seem to get right.

I was following a tutorial by WalkerBoy Studios about Cameras. I have an error:

Assets/2D Mario Assets/Scripts/cameraSmoothFollow2D.js(24,61): BCE0023: No appropriate version of 'UnityEngine.Mathf.SmoothDamp' for the argument list '(float, float, UnityEngine.Vector2, float)' was found.

 var cameraTarget            :    GameObject;            
     
 var smoothTime                :    float = 0.1;        
 var cameraHeight            :    float = 2.5;        
 
 var cameraFollowX            :    boolean =  true;    
 
 var velocity                :    Vector2;
     
 private var thisTransform            :    Transform;
         
 function Update ()
 {
     if( cameraFollowX){
     
         thisTransform.position.x =  Mathf.SmoothDamp(thisTransform.position.x, cameraTarget.transform.position.x, velocity, smoothTime);}}
  

I know that the problem is the 3rd variable being a Vector 2. But in the tutorial it makes velocity a Vector 2. But how would fix this problem but have the same result as the tutorial?