Just delete the method.
//DELETE THIS LINES:
android {
compileSdkVersion 21
buildToolsVersion '21.1.2'
}
Try to compile again. It should work now.
Source : https://medium.com/@marcuspereira/solving-the-gradle-dsl-method-not-found-android-in-android-studio-6e5ab499bd3#.u1lxecq41