Android Studio 2.1 Cannot resolve symbol 'R'

Question

Android Studio 2.1, Java V8 Update 91, Gradle 2.1.0 .

I know that this questions has been asked many times here, but none of the recommendation solutions works out.

Answer 1

Try syncing the project, go to Tools > Android > Sync Project with Gradle Files

UPDATE

If you're using Android Studio 3.3, use the Sync Project with Gradle Files icon