This error message means we are able to find an account using that email and is also indicating 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. Go back to the first screen that you get to when opening the app and press Log in instead of Get started.
2. Enter another email that you have that has not yet been connected to a Lifesum account. 
If none of these steps work, please reach out to us here. We can either help you reset your password etc or help you delete the old account if you reach out from the connected email.