This error message means that you have already registered an account with the email address in question and that you are not on the login page but on the page where you register a new account (which is not possible in this case as the email address is already connected to an account).
1) Close the page > Go to Me > Settings in your app and scroll down to the end of the page. Then log out of the app.
As soon as you have logged out you will come to the start screen. In order to log in, please click at the bottom of the screen on "Log in". Here you can log in with your email address and access your account.
2) Contact us in order to get the old account deleted. That will set the email address free again and you will be able to register a new account with it.