Adding item to list error?

there:

 void Start () 
      { 
          public Wall prefab;
          public Floor prefab;
          int ArraySize = 20;        //Actual array size will be 21x21 as it includes 0.
          Random rnd = new Random();
          int[,] MapArray = new int[ArraySize, ArraySize];        //Creates a 21x21 2D array (for future reference it may help to think of the map as a graph consisting of x and y coordinates).
          List<int> listxCoodinates = new List<int>();        //Creates an empty list to contain all the x-coordinates.
          List<int> listyCoodinates = new List<int>();        //Creates an empty list to contain all the y-coordinates.
          for (int x = 0; x < ArraySize; x++)        //This loop simply adds the number of elements in the array length-wise or width-wise to 2 separate lists, I know this would not work if the MapArray had different amount ofx-coordinates to y coordinates. I may fix this later.
          {
              listxCoordinates.Add(x);    //This is a list containing all the x-coordinates of the map.
              listyCoordinates.Add(x);    //This is a list containing all the y-coordinates of the map.
              MapArray[x, 0] = 1;            //This is for creating the outer walls of the map (1 represents a wall, 0 represents the floor)
              MapArray[x, MapArray.GetLength(0) - 1] = 1;
              MapArray[0, x] = 1;
              MapArray[(MapArray.GetLength(1) - 1), x] = 1;        
  
          }
  
          var RandomxOry:int = Random.Range(0, 2);
          listxCoordinates.Remove(0, 1, MapArray.GetLength(0), (MapArray.GetLength(0) - 1));
          listyCoordinates.Remove(0, 1, MapArray.GetLength(1), (MapArray.GetLength(1) - 1));
          int RandomxCoordinate = rnd.Next(listxCoordinates.Count);        //This gives a random index, not a random number in the list for the bot side of the map.
          int RandomyCoordinate = rnd.Next(listyCoordinates.count);        //This gives a random index, not a random number in the list for the left side of map.
          if (RandomxOry == 0)    //0 represents x
          {
              MapArray[listxCoordinates[RandomxCoordinate], 1] = 1;
              if (listxCoordinates[RandomxCoordinate] == 0)
              {
                  listxCoordinates.RemoveAt(RandomxCoordinate, RandomxCoordinate + 1);    //Removes item  at index in list 0 and 1. 
                  for (int x = 1; x < MapArray.GetLength(1); x++)
                  {
                      if (MapArray[listxCoordinates[RandomxCoordinate], x + 1] == 0)
                      {
                          MapArray[listxCoordinates[RandomxCoordinate], x + 1] = 1;
                      }
                      else
                      {
                          x = MapArray.GetLength(1);
                      }
                  }     
              }
               // HERE YOU MISSED TO ENCAPSULE THE ELSE-IF ()
              else if (listxCoordinates[RandomxCoordinate] == listxCoordinates.Count - 1)
              {
                  listxCoordinates.RemoveAt(RandomxCoordinate, RandomxCoordinate - 1);
              }
              else 
              {
                  listxCoordinates.RemoveAt(RandomxCoordinate, RandomxCoordinate + 1, RandomxCoordinate - 1);
              }
          }
          if (RandomxOry == 1)
          {
              MapArray[1, listyCoordinates[RandomyCoordinate]] = 1;
              if (listyCoordinates[RandomyCoordinate] == 0)
              {
                  listyCoordinates.RemoveAt(RandomyCoordinate, RandomyCoordinate + 1);    
              }
              // HERE YOU MISSED TO ENCAPSULE THE ELSE-IF ()
              else if (listyCoordinates[RandomyCoordinate] == listyCoordinates.Count - 1)
              {
                  listyCoordinates.RemoveAt(RandomyCoordinate);
                  listyCoordinates.RemoveAt(RandomyCoordinate - 1);
              }
              else 
              {
                  listyCoordinates.RemoveAt(RandomyCoordinate);
                  listyCoordinates.RemoveAt(RandomyCoordinate + 1);
                  listyCoordinates.RemoveAt(RandomyCoordinate - 1);
              }
          }
          for (int r = 0; r < MapArray.GetLength(0); r++)
              for (int c = 0; c < MapArray.GetLength(1); c++)
              {
                  if (MapArray[r, c] == 1)  // if the value of this index is 1
                  {
                      (Wall)Instantiate(yourPrefab, new Vector3(r, c, 0), Quaternion.identity);
                  }
              }
      }