You're signed in as
an account with no email address
Access is granted to one specific email address. If you were invited at a different one — a work address rather than a personal one, say — sign in with that address instead and you'll come straight in. Otherwise ask whoever invited you to grant access to this address.
Taking an assessment doesn't need an account. If you were sent a link to answer one, or to see a team's results, open that link — it works on its own.