I tried multiple boot systems, from 10.8 to 10.15. Although there are changes over time in that "resetpassword" app from the terminal, the only real differences are appearance, as all need the same two selections: volume where the users are located, and then the user account where you wish to change (reset) the password.
When you enter "resetpassword", you first choose the volume where your user account is located, then you would be offered, depending on the system version for the recovery system, choice for the volume where the user is located, then a choice for the specific user that you want to reset.
Are you choosing the correct volume first?
(Is that volume encrypted by File Vault?)