Initializing Games Client in Android

I had the same problem initially. What I had to do was look at the full, unfiltered LogCat log. There, I saw the message:

GamesIntentService(17929): Using Google Play games services requires a metadata tag with the name "com.google.android.gms.games.APP_ID" in the application tag of your manifest

So, assuming you created an entry in your strings.xml called app_id, try adding the following to your AndroidManifest.xml under the <application> tag:

<meta-data android:name="com.google.android.gms.games.APP_ID"
        android:value="@string/app_id" />

You can find your APP_ID in the Games Services tab

Complementing Hetabiquo response, if you also use Cloud Save service in your game you also must to update your application's AndroidManifest.xml by adding the following meta-data tag inside <application> tag scope

<meta-data android:name="com.google.android.gms.appstate.APP_ID" android:value="@string/app_id" />