Firmware password

Get the previous owner to tell you the password.

Or, from: How to set a firmware password on your Mac - Apple Support

" If you forgot your firmware password. If you can't remember your firmware password or passcode, schedule an in-person service appointment with an Apple Store or Apple Authorized Service Provider. Bring your Mac to the appointment, and bring your original receipt or invoice as proof of purchase."

The previous owner needs to do this:

What to do before you sell, give away, or trade in your Mac - Apple Support

You've got yourself a doorstop. The purpose of the firmware password is to provide security and it is working.

If a firmware password could be broken then there would be no reason to provide firmware password protection.