How to fix "No MonoBehavior scripts in the file, or their names do not match the file name" Unity C# error?

Question

Unity

Here is my script:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Player : MonoBehaviour
{
    public float speed;

    private Rigidbody2D rb;
    private Vector2 moveInput;
    private Vector2 moveVelocity;

    void Start()
    {
        rb = GetComponent<Rigidbody2D>();
    }

    void Update()
    {
        moveInput = new Vector2(Input.GetAxisRaw(Horizontal)
        moveVelocity = moveInput.normalized * speed;
    }

    void FixedUpdate()
    {
        rb.MovePosition(rb.position + moveVelocity * Time.fixedDeltaTime);
    }
}

Answer 1

If you throw an error message into Google translator, then everything becomes clear:

The file does not contain MonoBehaviour scripts or their names do not match the file name

This is a Unity-specific error, so the C# tag is not needed here.
Let me remind you that it is not clear why the unit calls classes scripts