applications – Unresolved reference: app_toolbar

I’m with a little problem. l’m beggining in Android development and in the use of Android Studio. I’m brazilian and I was watching a tutorial video:

My problem is in 6:10…

The “app_toolbar” id that is in “acitivity_main.xml” appear as “unresolved reference: app_toolbar” in “MainActivity.kt” and I don’t know what I should do. I just looked for it, but nothing help me. I did all the same that the guy of this video…