Users have keychains.
Users can also share system-wide keychains.
The login keychain is usually one such keychain, and usually the default, and usually where mail account credentials are stored.
Keychains can get corrupted.
Keychain contents can also get deleted through user or app requests or errors, too.
The prompt is telling you the item sought (imap.mail.me.com, outlook.office.com) and the keychain (login).
The keychain password sought is by default and usually the login password for the current macOS logged-in user, though the login password and the keychain password can sometimes differ.
If you have looked in the appropriate keychain and find no matching item, then you will need to re-connect the Outlook client with the mail account including specifying the mail server passwords, or you will have to find an older copy of the keychain from your backups that still contains the password from prior to deletion or prior to corruption.
It’s usually easier to re-add the mail account in the mail client (Outlook), though I don’t know if that delete-and-recreate has any side-effects within Outlook. (There aren’t any adverse side-effects in the Apple Mail client.)