The Zotero app does not currently have an Undo function. That it’s on the menu is an artifact of the Firefox platform on which Zotero is built.

CMD-Z CTL-Z works for most fields if you haven't clicked outside that field (or changed case of the title or journal name fields). For example, With ahead-of-print ePubs, for my purposes I want the volume, issue, and pagination fields to all show "ePub" until the article is assigned to an issue. Sometimes the publisher will place useless information in the page field. When selecting the page data for replacement, sometimes I move too quickly and replace what is in the date field. I can easily CMD-Z to revert to the original value.