WARNING: No compatible versions of com.android.support:support-v4

My app has launched with Admob on August . There wasn't this issues at the time.

Now I'm trying to update of it with Unity 5.3.5f1. I get an error claims 'WARNING: No compatible versions of com.android.support:support-v4 required by...' as the above picture shows.

I googled about it, then the solution seemed to be 'updating Android support library'. But I can't find the 'support library ' in Android Studio2.2.2. I can't find the 'extra folder' that supposed to include library.

What am I missing? I updated Unity, android studio, sdk, and JDK. I can run the app, but I'm afraid of crushing that might happen at user's device if I release it with this warning.

Please someone give me any tips! I've been stacked here for days.