Microsoft releases “Insider” (Beta) preview builds of Word for Mac to allow users to test upcoming new features. Occasionally, these preview builds (particularly the “Insider Fast” builds) cause Zotero's Word Add-in to work incorrectly or not at all. If you're using a Word Insider build and you've followed other troubleshooting steps, but your issue remains, try to downgrade to the latest stable version of Word to see if this resolves your issue.
Follow these steps to downgrade to the stable version of Word:
The Zotero tab should now appear in Word, and the plugin should work correctly.