The only two passwords I know of are the macOS admin user account password and a Filevault password if you turned on Filevault to encrypt the drive. Normally macOS does some behind the scenes "magic" so you only need to use your macOS admin login password to unlock Filevault & to log into your user account.
Installing a macOS upgrade should just install if you are going from an old OS to a newer OS especially if you are running the Monterey installer while booted to the internal macOS boot drive normally even if Filevault is enabled.
Is your macOS user account one with "admin" privileges? If not, then that is the problem and the question becomes how did it lose its admin privileges?
If you are upgrading the OS by booting into Internet Recovery Mode (Command + Option + R), or from a bootable macOS Monterey USB stick, then if Filevault is enabled on the internal drive you may first need to launch Disk Utility to unlock & mount the internal drive before you can install Monterey to it.