I would suggest a restart of the machine then Open EndNote first and then open a blank Word document to see if the issue persists.
You may need to try the steps to reset the Word preferences.
https://support.clarivate.com/Endnote/s/article/EndNote-Resetting-Word-Preferences?language=en_US
If the above does not help or if you have trouble with the steps then please contact Technical Support directly:
1-800-336-4474 option 4 then 1
or
We are here to help whenever you run into issues.