If you are getting the above error when logging into the sync app, this would mean you have/used to have 2 Factor Authentication associated with your login.
Unfortunately the sync app does not allow you to use an email address that has, or used to have 2FA enabled.
To clear this error, you will need to use another email address for the sync app. You can create a new user with another email in the "All People" tab.
You can use another user's login that has always had 2FA disabled.