Admob InterstitialAdRequst fails only for Android. Please help me.

I use GoogleMobileAds Asset.
Admob InterstitialAdRequst fails only for Android.
And App crashes.
IOS succeeds.
I tried a lot of solutions, but I don't solve it.
I tried Resolve and ForceResolve and Version Handler Update.
UnityRedErrorLog is nothing.
Gradle Build System occurs another problem.
So I would like to use Internal Build System.
Please help me.
**Version**
Unity2018.1.6f1
**Platform**
Android
**Build System**
Internal
**Android Studio logcat**> Unity: AndroidJavaException:> java.lang.ClassNotFoundException:> com.google.android.gms.ads.AdRequest$Builder> java.lang.ClassNotFoundException: com.google.android.gms.ads.AdRequest$Builder> at java.lang.Class.classForName(Native> Method)> at java.lang.Class.forName(Class.java:308)> at java.lang.Class.forName(Class.java:272)> at com.unity3d.player.UnityPlayer.nativeRender(Native> Method)> at com.unity3d.player.UnityPlayer.c(Unknown> Source)> at com.unity3d.player.UnityPlayer$e$2.queueIdle(Unknown> Source)> at android.os.MessageQueue.next(MessageQueue.java:211)> at android.os.Looper.loop(Looper.java:122)> at com.unity3d.player.UnityPlayer$e.run(Unknown> Source)> Caused by: java.lang.ClassNotFoundException:> Didn't find class> "com.google.android.gms.ads.AdRequest$Builder"> on path: DexPathList[[zip file> "/data/app/xxx.xxx.xxx-2/base.apk"],nativeLibraryDirectories=[/data/app/xxx.xxx.xxx-2/lib/x86,> /vendor/lib, /system/lib]]> at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)> at java.la