There is currently a known issue with certain versions of Microsoft Word whereby the Cite While You Write (CWYW) Add-in does not appear in Word.
When you run EndNote and try to use the Microsoft Word Add-in feature, it will not add the CWYW Add-in to Word.
There is an additional EndNote CWYW Plugin available on the EndNote online website. This has been downloaded and is attached to this article on the right hand side.
Extract the zipped file to your local machine and then run the EndNotePlugins.exe to install the Plugin.
Refer to the EndNote website https://www.myendnoteweb.com/EndNoteWeb.html?func=showcwyw&