Error: Execution failed for task ':app:clean'. Unable to delete file

If you are using Android Studio 2.0 Beta, this issue might appear (more likely if you are working on NTFS filesystem) and it seems like the "Instant Run" is the culprit. Search for "Instant Run" in settings and uncheck the box.

I have filed an issue on the bug tracker.

I killed all the java TM processes in the task manager and it let me to rebuild

Try File -> Invalidate Caches and Restart

This worked for me