Why RSVP No can be absent from a preview

In GiveCampus Events, available controls depend on your permissions and the interface used by your institution.

Enabling RSVP No is only one condition. The landing-page button also requires the event to be active or draft, within its registration window and to have remaining ticket capacity. It appears in the event information area using the configured button text. Check those conditions and the actual preview/public page before assuming the setting did not save; a draft preview is not exempt from every timing/capacity condition.