Android Error: "Unity Ads cannot show ads: webapp not initialized"

Hi everybody!

Unity Ads shows a black screen on Android (not Unity for Android, an Android game), the error shown is this:

I/UnityAds(17755): com.unity3d.ads.android.UnityAds.canShow() (line:242) :: Unity Ads cannot show ads: webapp not initialized

I think it has something to do with the "xml layout", I have my own "res/layout" files so I copied the Unity Layout there, but it seems not to read them.

What I have to do?

Thanks.