error CS0201 (2d roguelike)

"Assets/Scripts/BoardManager.cs(65,26): error CS0201: Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement"

That error redirects me to line 65: ( gridPositions.RemoveAt [randomIndex];)

But I can not find the error. All code below:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System;
 using Random = UnityEngine.Random;
 
 public class BoardManager : MonoBehaviour {
 
     [Serializable]
     public class Count{
         public int minimum;
         public int maximum;
 
         public Count(int min, int max)
         { // construct
             minimum = min;
             maximum = max;
         }
     }
 
     public int columns = 8;
     public int rows = 8;
 
     public Count wallCount = new Count(5,9);
     public Count foodCount = new Count(1,5);
 
     public GameObject exit;
     public GameObject[] floorTiles;
     public GameObject[] wallTiles;
     public GameObject[] enemyTiles;
     public GameObject[] foodTiles;
     public GameObject[] outerWallTiles;
 
     private Transform boardHolder;
     private List <Vector3> gridPositions = new List<Vector3> ();
 
     void initialiseList(){
         gridPositions.Clear();    
         for (int i = 1; i < columns - 1; i++) {
             for (int j = 1; j < rows -1; j++){
                 gridPositions.Add (new Vector3 (i, j, 0f));
             }
         }
     }
 
     void boardSetup(){ // to set outerWall and floor of the game
         boardHolder = new GameObject ("Board").transform;
 
         for (int x = -1; x < columns + 1; x++) {
             for (int y = -1; y < rows + 1; y++) {
                 GameObject toInstantiate = floorTiles [Random.Range (0, floorTiles.Length)]; // instantiate random floor tiles in the range
                 if (x == -1 || x == columns || y == -1 || y == rows) {
                     toInstantiate = outerWallTiles [Random.Range (0, outerWallTiles.Length)]; // to instantiate random outer wall tiles in the range
 
                     GameObject instance = Instantiate (toInstantiate, new Vector3 (x, y, 0f), Quaternion.identity) as GameObject;
                     instance.transform.SetParent (boardHolder);
                 }
             }
         }
     }
 
     Vector3 RandomPosition(){
         int randomIndex = Random.Range (0, gridPositions.Count);
         Vector3 randomPosition = gridPositions [randomIndex];
         gridPositions.RemoveAt [randomIndex];
         return randomPosition;
     }
 
     void layoutObjectAtRandom(GameObject[] tileArray, int minimum, int maximum){ // spawn the titles randomly
         int objectCount = Random.Range(minimum, maximum + 1); // para decidir quantas paredes por exemplo terá no nivel
         for (int x = 0; x < objectCount; x++) {
             Vector3 randomPosition = RandomPosition ();
             GameObject tileChoice = tileArray[Random.Range(0,tileArray.Length)];
             Instantiate (tileChoice, randomPosition, Quaternion.identity);
         }
     }
 
     public void setupScene(int level){
         boardSetup ();
         initialiseList ();
         layoutObjectAtRandom (wallTiles, wallCount.minimum, wallCount.maximum);
         layoutObjectAtRandom (foodTiles, foodCount.minimum, foodCount.maximum);
         int enemyCount = (int)Mathf.Log (level, 2f);
         layoutObjectAtRandom (enemyTiles, enemyCount, enemyCount);
         Instantiate (exit, new Vector3 (columns - 1, rows - 1, 0f), Quaternion.identity);
     }
 }