I don't have a "Not Junk" button

The Junk icon in the Mail toolbar will change its name to Not Junk when you select a Junk Mail item. When you click that Not Junk toolbar item with a selected Junk Mail msg, it is moved to the inbox of that Mail account.

Move the Non Junk mail to you Inbox.

It may take some time for it to Learn this persons' or Company e0mail address is not junk

Or

Use rules to manage emails you receive in Mail on Mac - Apple Support (CA)

Highlight an item in the Junk folder then the tiny arrow of the Jink icon points up instead of down & becomes Not Junk button.