Please watch this video for one of the ways to fix the issue of your Fitbit Charge 2 not getting paired back to Fitbit android app after unpairing it.
One of the reasons that may result in this pairing issue is that your Charge 2 tracker ran out of battery and was dead for quite some time before charging it back again. After charging the tracker shows an old date & time and fails to sync with the Fitbit app.
A soft restart (or reboot) to the tracker did not work and there is no factory reset available for Charge 2. So you have tried to fix the syncing issue by un-pairing the Charge 2 tracker from the app and then pairing it back. The tracker got unpaired from the app but when you tried to pair it back, the pairing process was getting failed after entering the 4-digit code/pin into the app.
If this describes the pairing & syncing issue your are facing with your Charge 2 then please go through this video and see if the process detailed in it will fix the problem for you.
