If using Microsoft Authenticator, you may receive the following error message when trying to remove an account:
You need to disable backup to delete <account>.
This can happen if you are trying to delete the account that is also acting as the Recovery Account for authenticator backups. To resolve, go into the Microsoft Authenticator settings. Find the section titled Backup, then disable backup. For example, on iOS this will be via iCloud Backup.
It will then ask whether you want to delete your backup: You will no longer be able to recover your accounts.
Click OK. You will receive confirmation that Backup successfully deleted with a Cancel button.
Go back to the main Authenticator screen. You will now be able to delete the account from Microsoft Authenticator.