From EndNote’s website:
“Cite While You Write is not currently compatible with Office 2016 on Macintosh.
We are actively developing a patch for EndNote X7 to fix this compatibility issue. We anticipate this free update to X7 to be ready by the Fall. We understand EndNote’s importance in completing your work and apologize for any inconvenience caused. To support Mac workflows as we develop the patch, we have the following recommendations to help Microsoft Office 365 users continue to create formatted citations and bibliographies in Microsoft Word.
It is possible to have both Office 2011 and Office 2016 on the same computer. If you want or need to install Office 2016, the recommendations outlined below will enable your continued use of EndNote to create formatted citations and bibliographies in your Word documents.”