If you are changing the email address for your login and see this error message, it means another login is currently using this email.
If you own this email and want to make it available again, there are a few options:
Free up the email yourself
Log in as that user, using that email address. Change the email on that login to another, unused email, thereby freeing up the desired email for use.
Request deletion of login using desired email
Contact Support to request deletion of that login. If you previously used that login, double check that there is no important information that will be lost. We will need to confirm ownership of the login before proceeding.
Swap logins moving forward
Contact Support to swap places between logins. Instead of reclaiming the email from another login, use that login moving forward and abandon the current login. Since each login has different permissions, we will need to review all the settings that need to be transferred over.