I have several different websites, and when in SiteBuilder under Page Settings --> Advanced Settings, the set a password option is only there on some of my website and simply missing on others. I have attached two photos to show the difference.
Sites that do not use Master Templates will always have the password field available on every page. However newer sites that were created with SiteBuilder Plus and include the Master Templates, have a feature where this can be turned on or off. If you need to set a password for a page, but don't see the option, click the "Background, Links, Footer" tab, and then you will see a button that says "Set for this page". After selecting that button, then go back to the "Advance Settings" tab, and it will show the password field.