When occur Unable to resolve target 'android-xx' error

It's often occurs this error when we developed on different Eclipse ( or Android SDK Manager)

refer to the stackoverflow:

Android error in eclipse: Unable to resolve target 'android-xx'

and 

Android change SDK version in Eclipse? Unable to resolve target android-x

Download the target SDK in Android SDK Manager can resolve this problem. 

簡單而言是因為使用不同的 SDK 版本或是 SDK Manager 裡沒裝對應的 SDK 導致，重新安裝對應的版本後再 Clean Project ( Eclipse > Project > Clean...)即可。