[Admob] Interstitial script The type `InterstitialAd' does not contain a constructor

Hello everyone, how are you? I’m fine.
I’m trying to write the scripts for monetizate my game with Google AdMob.
I have to make two scripts, one for my BannerAd and the second for the interstitial.

This is my script for interstitialAd. I have written the code that loads and shows the ad.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using GoogleMobileAds.Api;

public class InterstitialAd : MonoBehaviour {
private InterstitialAd interstitial;
 
private void requestInterstitial() {
    //Los siguientes adUnitId son anuncios de prueba, antes de publicarlo debo reemplazar adUnitId con el id del bloque de anuncios
    #if UNITY_ANDROID
        string adUnitId = "ca-app-pub-3940256099942544/1033173712";
    #elif UNITY_IPHONE
        string adUnitId = "ca-app-pub-3940256099942544/4411468910";
    #else
        string adUnitId = "unexpected_platform";
    #endif
   
    // Initialize an InterstitialAd. //Id Del bloque de anuncios
    this.interstitial = new InterstitialAd("ca-app-pub-3940256099942544/1033173712");
    // Use this for initialization
     // Create an empty ad request.
    AdRequest request = new AdRequest.Builder().Build();
    //Load the interstitial with the request interstitial
     this.interstitial.LoadAd(request);
}
    //ShowAd
private void showInterstitial() {
 
    if (Time.time % 45 == 0 && Time.timeSinceLevelLoad > 60 && this.interstitial.IsLoaded()) {
                this.interstitial.Show();
               
            }
}
       
 
 
}

But I have an error that says

Assets/Scripts/InterstitialAd.cs(20,25): error CS1729: The type InterstitialAd' does not contain a constructor that takes 1’ arguments

Assets/Scripts/InterstitialAd.cs(25,24): error CS1061: Type InterstitialAd' does not contain a definition for LoadAd’ and no extension method LoadAd' of type InterstitialAd’ could be found. Are you missing an assembly reference?

Assets/Scripts/InterstitialAd.cs(30,79): error CS1061: Type InterstitialAd' does not contain a definition for IsLoaded’ and no extension method IsLoaded' of type InterstitialAd’ could be found. Are you missing an assembly reference?

Assets/Scripts/InterstitialAd.cs(31,23): error CS1061: Type InterstitialAd' does not contain a definition for Show’ and no extension method Show' of type InterstitialAd’ could be found. Are you missing an assembly reference?

Hope you can help me ! Thanks, Mauro

I continue with the problem… Hope someone can help me ! Thanks, from Argentina.