If you hover your mouse next to the title “Favorites” there is a Plus showing up on the right.
If you click there, you could add several Mailboxes, with Remind Me among them.
Well I asked bing chat and it provided the following answer:
If you’re not seeing the Remind Me mailbox in Apple Mail, it could be because it’s not enabled. You can enable it by going to Mail > Preferences > Viewing > Show Reminders. If that doesn’t work, you can try restarting your Mac or updating your operating system¹.
That was the Way, I got my first iPhone, when my HTC WindowsPhone ended up in the corner next to the door, and I switched to iOS to get rid of all the problems…
And this was the first, and only time in my (now rather long) live, I threw something around, because it was killing my last nerves…