Mojave password recovery

Perhaps the person who sold you the Mac has already created a user account on it.

If that's the case, ask them what password they set.

Personally, with a 'new' secondhand Mac, I always erase it entirely and set up a new account from scratch; there's no way of knowing what might have been placed on the machine before you received it.

Ideally boot up holding down the Command and R keys to enter recovery mode

Select Disk Utility then erase the drive. Quit Disk Utility

Then reinstall macOS