How to display ads with Vungle?

Hi, I´m working on App and it seems like I have integrated Vungle correctly because I can see it reflected on my dashboard, the problem is when I call for the ads the app crashes or simply doesnt display the ads

This is part of my code where I want it to work

I have imported these

public function main()

  {

  menuStart.showStartScreen();

  menuStart.addEventListener("START_PLAYING", startGame);

  menuStart.addEventListener("GO_OPTIONS", openOpt);

  Vungle.create([,"53dc06fd5d261a11060001a3"]);

  introChannel = introS.play();

  function openOpt(e:Event):void

  {

  menuStart.hideStartScreen();

  menuEnd.hideScreen();

  menuOpt.showScreen();

  Vungle.vungle.displayAd();

  popS.play();

  menuOpt.addEventListener("GO_BACK", goBack);

  }

And I have made all modifications to my xml according to Vungle

<application>

    <activity

      android:name="com.vungle.sdk.VungleAdvert"

      android:configChanges="keyboardHidden|orientation|screenSize"

      android:theme="@android:style/Theme.NoTitleBar.Fullscreen"

    />

    <service android:name="com.vungle.sdk.VungleIntentService"/>

  </application>

... and the permissions

Any idea why this might be happening?

Thanks in advance