This vid helps Fix FBBluetoothErrorDomain error 18 while trying to upgrade your Fitbit tracker firmware. Learn more?
Solution: Stage 1: Let your phone's Bluetooth forget your Fitbit device temporarily. Below are detailed steps: 1. Go into settings on your phone (e.g., iPhone) and select Bluetooth. 2. Look on the list of devices and see if any Fitbit device that you have is listed. If it is select the device and choose the option “forget this device”.
Next, tap the "Forget Device" button.
3. Turn Bluetooth off on your phone.
4. Get your own FitBit device in hand. Open the FitBit app on your phone.
As it syncs it should tell you that there is an update (refer to the below image). You have to select update now and this should end up downloading the new software as well as sync with your FitBit device.
Note: If the update doesn’t download the first time, definitely try it (forgetting your device) again. If the issue continues, do the following Stage 2. Stage 2: troubleshooting FitBit firmware updates. See detailed instructions@ goo.gl/hE4fRt "Why can't I update my Fitbit device?".