How to install endnote x8 into Microsoft word 2016/2017
Endnote x8 isn't always compatible with Word 2016.

Word had a major upgrade behind the scenes - the older version of endnote works with Word version 15.x and lower. A newer patched (fixed) version of endnote x8 works with Word version 16.x (and possibly back with some older Word, too).

I would update both Word to the latest version and endnote to the latest version and try the install again.

Details are here from the endnote vendor support:

https://support.clarivate.com/Endnote/s/article/EndNote-Word-2016-Mac-version-16-Compatibility?language=en_US