Mojave includes new Security & Privacy functions which can prevent EndNote from being able to correctly automate.  To correctly set these options...

1)  Go into the Apple menu
2)  Click System Preferences
3)  Click Security & Privacy
4)  Click the Privacy tab (along the top)
5)  Click the Automation options, along the left.
6)  Here, make sure that both "Microsoft Word" and "EndNote X#" are both turned on.
7)  Close and restart EndNote and Word.

With this automation turned off, the Cite While You Write functions will not work.

If you are seeing the EndNote X8/X9 or EndNote tab in the ribbon, but the commands are not functional or only beep or do nothing when clicked, some settings may need to be reset. Do the following to resolve this:

1. Create a compressed backup of the library.
2. Quit EndNote and Word.
3. Open Finder, hold down the "Option" key on your keyboard and go to the "Go" Menu and choose "Library" then open the "Preferences" folder. [Note: If the Library folder is still not visible, try holding down the "Shift" key instead of the "Option" key with macOS Sierra or later.]
4. Remove the "com.ThomsonResearchSoft.EndNote.plist" file.
5. Browse to is the "[Hard Drive Name] : Library : Application Support : Microsoft : Office365 : User Content : Startup : Word" folder.
6. Delete the "EndNote CWYW Word 16.bundle" and/or  "EndNote CWYW Word 2016.bundle" file.
7. Launch EndNote X8/X9.
8. Run the Customizer to install the Cite While You Write tools. With EndNote online, run the "Install Cite While You Write" application.