Tried to ad 1 FACT and it doesn’t appear in GUI, only GEDCOM.
Most likely you didn’t look far enough. In the drop down list it is called (in English) “Custom fact”. It and “Custom event” are always listed at the very bottom of the list, not in the alphabetical order the others use. This puts them in same place for all languages to avoid translation confusion, and means they always exist even if not selected in admin “Edit options”.