can not build apk file when i import admob

firstly,I can export apk file properly.
but when I improt admob pluging,it can not export, and show error.How could I fix this?
I am using r16b ndk.i wonder if my configuration is wrong.

detail below:
UnityEditor.BuildPlayerWindow+BuildMethodException: 2 errors
at UnityEditor.BuildPlayerWindow+DefaultBuildMethods.BuildPlayer (BuildPlayerOptions options) [0x00242] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:194
at UnityEditor.BuildPlayerWindow.CallBuildMethods (Boolean askForBuildLocation, BuildOptions defaultBuildOptions) [0x0007f] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:97
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr) (at C:/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:179)

Same for me… Unity is headache. Better to move on UE4,For me also same… Why unity so hard… I can’t understand if like that how beginner will work??? Headache