An annoying message, asking for permission to create a folder that mutt then never uses (unless your MTA is configured for Maildir, but that's another story).
To fix it I just added unset folder
to the end of /etc/Muttrc.
You probably will want to remove that if you ever decide to switch to Maildir.
MetaEd. "Suppresss "~/Mail does not exist" in mutt". Unix & Linux StackExchange, 4 May 2023, https://unix.stackexchange.com/questions/108097/suppresss-mail-does-not-exist-in-mutt.