Mac laptop master password problems
The master password cannot be used in place of any user's password, it can be used to reset any user's password from the login screen. Just try to log in with the wrong password, and it should show an option to reset it with the master password.

If you're not seeing that, and you have FileVault turned on, you may be looking at the pre-boot authentication screen instead of the login screen (they look very similar). The master password cannot be used a the FileVault pre-boot authentication screen, but a FileVault recovery key can (if you selected that option when enabling FileVault), or linked iCloud account can (if you selected the "Allow my iCloud account to unlock the disk" option instead). Alternately, if you have another (non-admin) account, you could log in with that at the pre-boot authentication screen, then log out to the regular login screen.