I had this exact same problem and just solved it by following the below all the way through Step 12.
1. Turn off any other BlueTooth Devices with FitBit App Installed
2. Remove the tracker from your Phone's Bluetooth settings, not from the app.
3. Restart Phone
(Still not working? Continue with steps 4-9)
4. Plug the charging cable into the USB port on your computer or any UL-certified USB wall charger.
5. Clip the other end of the charging cable to the port on the back of the tracker. The pins on the charging cable must lock securely with the port.
6. Make sure the button on your tracker is aligned with the button opening on the charging cable. You’ll know the connection is secure when the tracker vibrates and you see a battery icon on your tracker’s display. Your Charge 2 will begin charging.
7. Press and hold the button on your tracker for four seconds. When you see the Fitbit logo and the tracker vibrates, this means the tracker has restarted.
8. Unplug your tracker from the charging cable.
9. Retry syncing your tracker again.
(Still not working? Continue with Steps 10-12)
10. Remove all Fitbit Devices from Fitbit App on Phone
11. Repeat steps 1-9
12. Add Fitbit as New Device & Follow steps for Pairing
