Today I faced the same problem, my phone was not unlocking and asking for backup password (even though I could feel the haptic vibration feedback indicating finger scan is detected). Anyhow coming to soluton.
#1: Using samsung find my mobile service, you can log in & unlock your phone via website findmymobile[dot]samsung[dot]com
#2: try entering wrong backup password for 5 continous attempts till the phone pops a 30 sec time delay. After the time delay you can see an option "Unlock Via Google". You can enter the Google Id with which your samsung phone is registered with.
Note: for both the options your phone has to be connected to internet.
Luckily for me my phone was connected to the office wifi so I could instantly unlock from the samsung portal