Hi @Bcmarso. Welcome to the forums. When removing a Fitbit device from the app, it is important to take a second step — Go into your iPhone’s Bluetooth from the Settings screen. Remove the Fitbit devices from there, also.
Then, make sure your Blaze and Flex 2 are fully charged and try once more to add them to your app. If you are still having issues, tap here for more trouble shooting tips.