Unity Ads not working, how to fix?

Question

Unity

I'm using Unity 2020.3.15f2, connected Unity Ads, and wrote a small script:

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

public class AdManager : MonoBehaviour
{
    public void Start()
    {
        if (Advertisement.isSupported)
        {
            Advertisement.Show("video");
        }
    }
}

I am connected to the Internet and have completed all the necessary actions in the Dashboard. But nothing works, what is the reason?