Cannot resolve symbol 'menu' errors in Android Studio

With more than 20 years of experiencie i can tell you a very difficult solution for the menu problem on android studio...:

*********** Close the project an open it again ************************

I would have love to add this as a comment to the first answer but I can't for now; So when I had the problem using android studio 2.3. I clean the project and then rebuild:

Build --> Clean Project and Build --> Rebuild Project

Check menu_main.xml is in your res directory. After its not working means, Once, Clear Your project.