If the email address that was entered for login, is not an agent/contact in the account, then this error would be displayed. You could sign up for a new account, using the Sign Up option on the Portal, or ask to be added as an Agent from under Admin >Team > Agents in your account. Once this is done, you could log in to your Freshdesk Account.
If you continue to face issues with login, reach out to Freshdesk support through [email protected] to know the Agents on your Freshdesk account and we would assist you in getting over this instance.