It is likely that the email address was previously used to add a user but since been deleted.
Rather than adding them, you will need to undelete their account to reinstate them as a user.
Select My Account > Users.
Click on the drop-down list to filter to view 'Deleted Only' users.
Select 'Edit' for user followed by the 'Undelete' button to reinstate them.
You may also wish to select the 'Reset Password' option to generate a new password creation link for them.