I had the exact same issue with a dell latitude 5480 where the trackpad I had tried everything I knew and eventually I had to reach out to dell for their warranty repair and they said all they did was reinstall windows 10. So I recommend backing up as much as you can and trying that

Did you check disk management to partition the drive?

Are you sure this is an authentic fitbit? it could be a knockoff that for what ever reason needed another cable