As I said in post #11, you have to add it by using Customize Toolbar - right-click anywhere in the toolbar and you'll see "Customize Toolbar" at the bottom of the popup. Choose that, and then drag to your toolbar what you want from the "Drag your favourite items to the toolbar ...." section that appears. In this case the Junk icon. Then when you highlight a message the app has decided is junk, the Junk icon will turn into a Not Junk icon, which you can click if you disagree. However ...
This is a sidetrack. My main point in starting this thread was that, even if you get the Junk/Not Junk icon, and even if you start marking stuff the app says is junk as not junk, the post-Sierra Mail app is not learning this as it should, and as it always has done prior to Sierra.