The reason why you get the popup asking for the registration of an email is coming up as the account you are currently logged into does not have a valid email address or linked registration with Facebook yet - you previously started the process of setting it up completely but somehow the process was not finished correctly.
That means that the account you are NOW logged into does not have a registered email address or functioning Facebook login.
If you now try to register an email and the system says that the email is already in use
This means that you have at least two different accounts and one registered with the email in question. You can solve it as follows:
1) use another email address
2) contact support with the email address you would like to use and ask them to delete the other account and let you know when that is done so you can continue with the registration
3) choose Facebook as the registration method