Please note that the below problem has been addressed in the X9.1.1 update for Mac. If you want to update start EndNote and go to EndNote X9>Check for updates...
After updating to EndNote X9.3.1
Please update to Endnote X9.3.2 were this issue has been addressed, Update to version EndNote X9.3.2 for Mac
If you try to convert a library saved as an enlp-file you will get this message. Please see this article for help Error converting enlp-libraries.
After updating to EndNote X9.1
Please note that the below problem has been addressed in the X9.1.1 update for Mac. If you want to update start EndNote and go to EndNote X9>Check for updates...
We have been receiving some reports from Mac users that there is an issue using Direct Export on the Mac after update to X9.1. It appears to be limited to direct export files with a .enw file extension. The development team is working on a solution/update to solve this problem.
A workaround in EndNote X9.1
Instead of double-clicking the export file, we recommend using the EndNote: File > Import method to import the file. Please choose under options the filter "EndNote Import" for Google Scholar. Other sources may be "Reference Manager (RIS)".
Another solution (direct export working) for Google Scholar is to change your import settings from EndNote to RefMan (from .enw files to .ris files).
scholar.google.com > Hamburger menu (three lines, top-left) > Settings > "Show links to import..." = RefMan.
Other reason for the message
Please note that you can also receive this message if the EndNote library is stored in a "hostile" location. Read this article for more information Storage location for Endnote libraries