05-05-2016 06:43
05-05-2016 06:43
05-06-2016 04:17
05-06-2016 04:17
Welcome to the Community @BarbaraWonko. If you are receiving an error 400 at the moment of setting up your tracker, I recommend checking if your iOS Fitbit app is updated by following the steps that @MarreFitbit provide in This Thread. If it is and you keep receiving this error, you can set it up from your Windows 10 device by following These Steps.
I hope this helps, let me know the outcome.