Word 2016 version 16.7 is actually the current Fast Track Office Insider Update. The production release of EndNote X8.1 is not currently compatible with this Office Insider release, but we announced our EndNote X8.2 Mac Beta 3 weeks ago. If you are eligible, you can sign up here: http://endnote.com/betasignup
Alternatively or if you are not eligible for the Beta, you can downgrade Word 2016 to the production release of version 15.39.