endless building for Xamarin Forms --> android project

1. Just added a Blank Xamarin.Forms App
2. Set the .Droid project as startup project
3. Build the .Droid project ---> The building does not finish, seems keep building for ever

--------- Environment:---------------------------
On OS X

Xamarin Studio
Version 5.9.7 (build 22)
Installation UUID: 42eb780a-effa-47a6-b09d-087b776ae11b
Runtime:
Mono 4.0.4 ((detached/d481017)
GTK+ 2.24.23 (Raleigh theme)

Package version: 400040004

Apple Developer Tools
Xcode 7.0.1 (8228)
Build 7A1001

Xamarin.iOS
Version: 9.0.1.29 (Trial Edition)
Hash: 1d27ac2
Branch: master
Build date: 2015-09-25 18:08:44-0400

Xamarin.Android
Version: 5.1.8.0 (Trial Edition)
Android SDK: /Users/user/Library/Developer/Xamarin/android-sdk-macosx
Supported Android versions:
4.0.3 (API level 15)
4.4 (API level 19)
5.0 (API level 21)
--------- Environment:---------------------------