Hello @LizR225 ,
This messaging is because the people in question are currently using the email addresses you're trying to add as usernames for other accounts they're on. They'll need to login to the other accounts they're on, and update their usernames on those other accounts so that you can add their emails to your account.
If you they further assistance with updating their other accounts' usernames, I'd advise having them call general support to provide their emails, confirm security permissions live, and work with the agent to administratively update those other usernames.
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