Check the box in front of the Cite While You Write option
Click next until the Customizer is finished.
Then open Word and check the Ribbon for the EndNote tab
Alternatively, try the following:
Open a Word document.
Go to Tools > Templates and Add-Ins.
You should see the below EndNote files listed under "Global Templates and Add-ins"
EndNote CWYW Word 2016.dotm
and EndNote CWYW Word 2016.bundle
or EndNote CWYW Word 16.bundle
If any of these file(s) is unchecked, check it, and click OK.
Close and re-open
If you are still not seeing the EndNote tab, follow these steps:
Close Word and EndNote.
Open Finder>Applications>EndNote 20>Cite While You Write>EndNote CWYW Word 2016.bundle
Copy the file "EndNote CWYW Word 2016.bundle" by highlighting the file and going to Edit>Copy
Open Finder>Go>Computer>Macintosh Hard Drive. Then follow the path Library>Application Support>Microsoft>Office365>User Content>Startup>Word and paste the file "EndNote CWYW Word 2016.bundle" by going to Edit > Paste
Start Word, go to Word>Preferences and click on File Locations.
If the Startup folder is set to any location, highlight Startup and click on Reset
Click OK and close Word
Start EndNote and then start Word.
Clarivate have some instructions for solving problems with the EndNote toolbar and various versions of Word available here.