Not sure what you mean… Is it that you're using Unified Folders and would expect the account to appear under the unified Inbox? Do you have other mail accounts in Thunderbird? Do you see the "phantom" account in the folder pane if you choose View > Folders > All Folders?

Anyway, the place to look for any account and manage it would be the Account Settings rather than the folder pane.

He means when using burger>New Account>Existing email entering the details and clicking [Done] this error message comes up and the new email account is not added. Deleting the Thunderbird data solves this, but that is NOT an option as that also deletes address books and current IMAP settings (including which emails have been read). When we were trying to add the new account, Thunderbird tried automagically to use the wrong server. When we entered the correct server we started to get the error message. Clearly there is a config error regarding the IMAP (incoming) server which total deletion removes (typical Windows fix: reset everything regardless of the collateral damage). How can we just remove the incorrect IMAP server setting and enter the correct one?