Robot repair script, error bce0048

I've been getting several errors for this script, but I knocked it down to just this one, object' does not support slicing. Line 65,26. This question has been answered before, but the other answers haven't helped me on this part.

 #pragma strict
 var cols:int = 4;//the number of columns in the card grid 
 var rows:int = 4;//the number of rows in the grid
 var totalCards:int = cols*rows;
 var matchesNeededToWin:int = totalCards * 0.5;//8 matches are needed to clear the board
 var matchesMade:int = 0;//at the start the player hasn't made any matches
 var cardW:int = 100;//Each card's width is 100 pixels
 var cardH:int = 100;//Each card's height is 100 pixels
 var aCards:Array;// store the cards created in this array
 var aGrid:Array;//keep track of shuffled, dealt cards in this array
 var aCardsFlipped:ArrayList;//stores the two cards that the player flips over
 var playerCanClick:boolean;//use this flag to prevent player from clicking buttons at certain times
 var playerHasWon:boolean = false;//stores whether or not the player has won
 var i:int;
 var j:int;
 
 function Start () 
 {
   playerCanClick = true;
   
   //initialize the arrays as empty lists:
   aCards = new Array();
   aGrid = new Array();
   aCardsFlipped = new ArrayList();
   
   for(var i=0; i<rows; i++)
   {
     var acrds:Array=new Array();
     
    for(var j=0; j<cols; j++)
     {
      acrds[j] = new Card();
     }
     aGrid[j]=acrds;
   }
 }
 
 function OnGUI () 
 {
   GUILayout.BeginArea (Rect (0,0,Screen.width,Screen.height));//this will be the width and height of the screen and it will start at the screen origin
   BuildGrid();//builds the grid of card buttons
   GUILayout.EndArea();
   print("building grid!");
 }
 
 class Card extends System.Object
 {
  var isFaceUp:boolean = false; //determines whether or not the card has been flipped
  var isMatched:boolean = false;//is set to true when the card has been matched
  var img:String;
  
  function Card()
  {
    img = "robot";
  }
  
 }
 
   function BuildGrid()
 {
  GUILayout.BeginVertical();
  for(i=0; j<cols; j++)
  {
   GUILayout.BeginHorizontal();//BeginHorizontal/EndHorizontal to make card buttons stacked horizontally instead of vertically
   for(j=0; j<cols; j++)
   {
     var card:Card = aGrid[i];
     if(GUILayout.Button(Resources.Load(card.img),
         GUILayout.Width(cardW)))//keeps the card from filling the width of the layout area
     {
       Debug.Log(card.img);
     }
   }
   GUILayout.EndHorizontal();
  }
  GUILayout.EndVertical();
 }