How to restore missing email subfolder button on MacBook?

I would like to be able to see the subfolders like: draft/Junk/sent/etc on my Mac. This is no problem on my iPhone but on my Mac book there is supposed to be a button in the top left of the screen, mine has been missing since I got this Mac book a couple of years ago.

How can I put the button where it is supposed to be?

Thanks in advance

John