From the docs/FAQ:
This message means an account with that email address, created before Build had Adobe ID authentication, already exists. You need to log in with that old account, and then link your Adobe ID to it. So on the sign in page, click where it says "Click here if you don't sign in with an Adobe ID". Sign in with either an email/password or using your Github account (however you would have created the old account). Once logged in, you will be prompted that "To continue using PhoneGap Build we ask you to please link an AdobeID to this account." and limited to the account settings page. Now Connect your Adobe ID, and you're done!
Let us know if you still have trouble.