Roguelike script class cannot be found

So, I am working on the roguelike project, and I cannot attach the BoardManager object to the Gamemanager object due to the script class not being found. I have included my code below, in case that helps

using System;
using System.Collections.Generic;
using UnityEngine;
using Random = UnityEngine.Random;

public class NewBehaviourScript : MonoBehaviour
{
    [Serializable]
    public class Count
    {
        public int minimum;
        public int maximum;
        public Count(int min, int max)
        {
            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[] foodTiles;
    public GameObject[] enemyTiles;
    public GameObject[] outerWallTiles;
    private Transform boardHolder;
    private List<Vector3> gridPositions = new List<Vector3>();
    void InitaliseList()
    {
        gridPositions.Clear();
        for (int x = 1; x < columns - 1; x++)
        {
            for (int y = 1; y < rows - 1; y++)
            {
                gridPositions.Add(new Vector3(x, y, 0f));
            }
        }


    }
    void boardSetup()
    {
        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)];
                if (x == -1 || x == columns || y == -1 || y == rows)
                {
                    toInstantiate = outerWallTiles[Random.Range(0, outerWallTiles.Length)];
                    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 LayOutObjectsAtRandom(GameObject[] tileArray, int minimum, int maximum)
    {
        int objectCount = Random.Range(minimum, maximum + 1);
        for (int i = 0; i < objectCount; i++)
        {
            Vector3 randomPosition = RandomPosition();
            GameObject tileChoice = tileArray[Random.Range(0, tileArray.Length)];
            Instantiate(tileChoice, randomPosition, Quaternion.identity);

        }
    }
    public void SetupScene(int level)
    {
        boardSetup();
        InitaliseList();
        LayOutObjectsAtRandom(wallTiles, wallCount.minimum, wallCount.maximum);
        LayOutObjectsAtRandom(foodTiles, foodCount.minimum, foodCount.maximum);
        int enemyCount = (int)Mathf.Log(level, 2f);
        LayOutObjectsAtRandom(enemyTiles, enemyCount, enemyCount);
        Instantiate(exit, new Vector3(columns - 1, rows - 1, 0f), Quaternion.identity);
    }
}

It looks like you made a script, renamed it to BoardManager, and forgot to change the class name itself.

Try renaming:

public class NewBehaviourScript : MonoBehaviour

To:

public class BoardManager: MonoBehaviour