Hello @Vanslover_357, thanks for joining us in the Community, it's great to have you on board! 
If your Flex 2 is not pairing to your phone I would like to suggest you follow these steps:
- Restart your Flex 2 and leave it plugged in and charging.
- Go to the Bluetooth settings on your phone and check if the tracker is on the list of paired devices. If it is, tap on it and select Forget this Device.
- Restart your phone.
- Make sure there are no other Bluetooth devices around that might interfere with the set up process.
- Make sure you have a strong Wi-Fi signal or enough cellular data.
- Open the Fitbit app and tap on the Account symbol at the top right corner of the screen, then tap on "Set up a New Device"
- Choose your tracker and follow the onscreen instructions to continue
I hope this can be helpful, please give these steps a try a couple of times and if your Flex 2 is still unable to finish the setup process, please feel free to reply.
Marco G. | Community Moderator, FitbitDid you find my post helpful? Vote for it or mark it as a Solution!