how do I erase all settings

Make sure there is no application running on the desktop:

The menu bar should look like this:

Click on Apple logo and the menu bar should show system preferences:

Click on System Preferences and the option to erase should appear:

Are you trying to erase the hard drive?

Do a backup, preferable 2 separate ones on 2 drives. Boot to the Recovery Volume (command - R on a restart). Run Disk Utility/First Aid. Reformat the drive using Disk Utility/Erase. Use Mac OS Extended (Journaled), then click the Option button, and select GUID. Quit Disk Utility, then re-install the OS.

Reinstall from macOS Recovery

ASC YouTube for how to reinstall macOS from macOS Recovery 

OS X Recovery

When you reboot, use Setup Assistant to restore your data.