Welcome to Android Central! It's difficult to use an Android device routinely if you forget the backup password, because the device will still ask for your backup password periodically in normal use, even when it detects your fingerprint successfully. Did you just set up this phone and then promptly forget the password?
You could do a hardware key factory reset:
SAMSUNG Galaxy A9 - How to Hard Reset my phone - HardReset.info. You'll then have to enter the Google account and password that was associated with the phone -- hopefully, you remember that.
