'IUnityAdsListener' could not be found

Full error: error CS0246: The type or namespace name 'IUnityAdsListener' could not be found (are you missing a using directive or an assembly reference?)

Unity 2019.4.17f1

Ads has been turned on

Package Manager shows Advertisement 3.5.2 installed

My C# script:

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

public class AdsManager : MonoBehaviour, IUnityAdsListener {

 string  googlePlayID        = "4055395";
 string  applePlayID         = "4055394";
 string  myVideoId           = "video";
 string  myRewardVideoId     = "rewardedVideo";
 string  myInterstatialId    = "Interstatial";
 string  myBannerId          = "Banner";
 bool    isTestGameMode  = false;

 // Start is called before the first frame update
 void Start()
 {
     Advertisement.AddListener (this);
     Advertisement.Initialize(googlePlayID, isTestGameMode);
     
 }

 public void DisplayInterstitialAds() {

     if (Advertisement.IsReady(myInterstatialId)) {
         Advertisement.Show(myInterstatialId);
         }
 }

 public void DisplayVideoAds() {

     Advertisement.Show(myRewardVideoId);

 }

 // Implement IUnityAdsListener interface methods:
 public void OnUnityAdsDidFinish (string surfacingId, ShowResult showResult) {
     // Define conditional logic for each ad completion status:
     if (showResult == ShowResult.Finished) {
         // Reward the user for watching the ad to completion.
         Debug.LogWarning ("You get a reward.");
     } else if (showResult == ShowResult.Skipped) {
         // Do not reward the user for skipping the ad.
         Debug.LogWarning ("You don't get a reward.");
     } else if (showResult == ShowResult.Failed) {
         Debug.LogWarning ("The ad did not finish due to an error.");
     }
 }

 public void OnUnityAdsReady (string surfacingId) {
     // If the ready Ad Unit or legacy Placement is rewarded, show the ad:
     if (surfacingId == myRewardVideoId) {
         // Optional actions to take when theAd Unit or legacy Placement becomes ready (for example, enable the rewarded ads button)
     }
 }

 public void OnUnityAdsDidError (string message) {
     // Log the error.
 }

 public void OnUnityAdsDidStart (string surfacingId) {
     // Optional actions to take when the end-users triggers an ad.
 } 

 // When the object that subscribes to ad events is destroyed, remove the listener:
 public void OnDestroy() {
     Advertisement.RemoveListener(this);
 }

}