This could have happened if you signed in with this address before (perhaps by mistake) or were using this email address to test creating ideas, voting or submitting tickets on your UserVoice site.
How to fix: delete the end user profile that is currently using the email address:
- Go to the front end of your UserVoice site— i.e. xyz.uservoice.com
- Click "Sign in" in the top right corner
- Enter the email address you want to use (the one that is currently taken) and sign in
- Once you're logged in, click the "Settings" link under your username
- Scroll down and click "Delete profile"
- Log back into the Admin Console and you can now use the email address that was previously taken