This Fall, Apple will release a new version of its operating system, called Catalina. The only version of EndNote that will work with Catalina is EndNote X9. All older versions of EndNote (i.e. X8 or older) will not work.
According to this notice from Clarivate (the company that produces EndNote), EndNote X9 is being updated to support Catalina and current users should install any updates from EndNote that appear when you launch the program. If you are running an older version of EndNote, you should upgrade to EndNote X9.
If you have questions about this issue or if you need assistance upgrading to EndNote X9, please contact Dana Medical Library and someone will be happy to assist you.