I don't know exactly why it is failing for you, but behind the reorganization of System Preferences into the new System Settings, most things still work the same as before.
Yes, they look different, and are grouped differently; but there is still Users & Groups in System Settings, and to delete a user account, one is supposed to click the circled i button and then click "Delete Account...". All in all, not that much different.
I understand that this process is failing.
I suggest you try the same while running your Mac in Safe Mode.
Doing it is not hard. If you have an intel Mac, start it and press and hold the Shift key until the Apple logo appears. If you have an Apple Silicon Mac, press and hold the power button until you get a screen showing you options to start, and choose Safe Mode.