Mac mail - All Inboxes option greyed out
I have accidently deleted the "All Inboxes" option in the sidebar. When I go to re-add this - by going to the Favourites, clicking on the +, the All Inboxes option is greyed out. How do I get it back so that I can put this back on the sidebar? Thanks in advance.
iMac 24″, macOS 14.6