This happens when you've already used your email address for another Evernote account in the past, even if you deactivated that Evernote account.
The solution for this is this to swap the email addresses on your accounts:
1. Sign in to the account with your desired email address at https://www.evernote.com
If the account has been deactivated, you can reactivate it here: https://www.evernote.com/AccountReactivation.action
2. Change the email address on that a