Call FitBit. They will check to see if it is still within warranty. Even if it isn't, they may offer you a discount on a new FitBit. That's how I got my Charge 2.

I have run into this when the backup I was trying to restore from was from a phone with a newer version of iOS than what was running on the phone I was restoring to. Try setting it up as a new phone first, then update iOS to the latest version. Once the phone has been updated, you should then be able to restore it from the backup.