When you contact Clarivate (EndNote) tech support they say that they do not handle the aspect of EndNote working with Pages because they don't control the plug-in. Basically, they put the onus on Apple for any problems and for support.
So, if like me you opened a Word document in Pages to work on it in Pages (I find it easier), I had to make sure I had downloaded the appropriate Endnote plugin (from the Apple or EndNote sites, nowhere else) based on your version of EndNote https://support.apple.com/en-us/HT204395 (as per VikingOSX). I'm pretty sure I did download the correct one the first time; however, I re-downloaded it just to be sure and reopen the Page version of the document in Pages again. Then my bibliography was added correctly versus when I had simply opened the Word document originally in Pages the first time.