Missing using directive or c# assembly reference?

Question

C++ / C#

using System.Collections.Generic;
using UnityEngine;

public class MovePlayer : MonoBehavior
{
public Transform player;
[SerializeField]
private float speed = 10f;
private void OnMouseDrag()
{
if (!player. lose )ERROR
{
Vector3 mousePos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
mousePos.x = mousePos.x > 2.5f ? 2.5f : mousePos.x;
mousePos.x = mousePos.x < -2.5f ? -2.5f : mousePos.x;
player.position = Vector2.MoveTowards(player.position,
new Vector2(mousePos.x, player.position.y), speed * Time.deltaTime);
}
}

}
ERROR, code for Unity
Missing using directive or assembly reference !

Answer 1

Let's try to learn and explain)))
Here is your code from the comments to another answer (to make it clearer what other people are talking about)

public class player : MonoBehaviour
{
public static bool lose = false;
private void OnTriggerEnter2D(Collider2D other) {
if (other.gameObject.tag == "bomb")
lose = true;
}

Now to the point.
In principle, a static variable can be accessed through the
ClassName.VariableName
WITHOUT any object references, that's why it is static.
But you have
a variable and a class named the same (not to mention the fact that the classes should be named with a capital)

//public Transform player;
public  SomeClassName someObjectRef;
//////
//bool flagFromStatic = player.lose;
bool flagFromStatic = SomeClass.staticVariableInClass;
bool flagFromClassObject = someObjectRef.nonStaticVariableInClass;

in general, it is clearly required to study the BASICS of the language and programming as such.
good luck

Answer 2

Transform has no lose field in principle. So either player must not be Transform, or there must be not lose.