Greetings daria_sim,
1. You’d want to start by quitting Word altogether.
2. After quitting Word, open a Finder window.
3. With Finder showing, click the “Go” menu at the top of your screen and choose “Computer”.
4. Open Macintosh HD.
5. Navigate to the following folder:
Library > Application Support > Microsoft > Office365 > User Content > Startup > Word
6. Take the “EndNote CWYW Word 2016.bundle” file out of the mentioned Word folder and place it inside your Trash.
7. Relaunch Word and open a document - you should no longer receive the mentioned error when working in Word.
Please let me know if this helps.