Both the 2016 and Office 365 versions of Word are compatible with EndNote 20's Cite While You Write (CWYW) Word plugin, but here are some common reasons that could cause CWYW disappear from Word and solutions for getting it back:
If you've recently updated Microsoft Word and/or EndNote Desktop:
Check both Microsoft and EndNote to make sure both are up-to-date. Sometimes updating one without updating the other can cause a mismatch between the two.
In EndNote, click on the EndNote tab in the menu bar and select the "Customizer..." option from the drop-down menu. Make sure that CWYW is checked off and click the Next button to follow through the rest of the customization process.
If you've recently installed other citation management plugins (e.g. RefWorks' RCM or WNC or Mendeley's Citation Plugin) in Word:
Uninstall or deactivate any citation management plugins that are not CWYW.
If none of the above suggestions work, we recommend contacting EndNote via phone or email, and their technical support staff can help you resolve the issue.