"All Inbox" option greyed out on Mail in my MacBook.

Suddenly my 'All inboxes' option has disappeared from my Mail app.

Having read 6x6's reply to the issue (back a few years now), I hovered over 'Favourites' and selected the '+', only to find 'All inboxes' is greyed out (see screenshot below).

Is there any other way I can get 'All inboxes' back?

[Re-Titled by Moderator]