It says my script doesn't derive from Monobehavior

using System.Collections;
using System.Collections.Generic;
using System.Security.Cryptography;
using UnityEngine;

public class Mover : MonoBehaviour
{
    private Rigidbody rb;
    public float speed;
    // Start is called before the first frame update
    void Start()
    {
        rb = GetComponent<Rigidbody>();
        rb.velocity = transform.forward * speed;
    }

    // Update is called once per frame
    void Update()
    {
        
    }
}

Help please?

I’m using Unity 2020.1.1f1, I’ve copied your code and I’ve pasted in a script of an empty project and it didn’t have errors. Check your Unity configurations because it works perfectly in my Unity.