You can save time by setting your Preferences for different functions including:
To open the Preferences menu go to EndNote 21 > Settings (from the menu at the top of your screen) or use the Command ⌘ and ,
Q: When I insert a citation to my Word, I receive this message: "This library is currently being used by someone else. Please try again later."
A: This is because your library is saved in your iCloud account.
You can check if your system is set to use the iCloud for the Desktop and Documents folder:
Click on the Apple icon in the upper left corner of your Mac's screen.
Select System Settings... from the dropdown menu.
Click [your name] at the top of the sidebar I(f you don’t see your name, click “Sign in with your Apple ID”, enter your Apple ID (or another email address or phone number on file), then enter your password. If you don’t have an Apple ID, you can create one.
Click on iCloud > click iCloud Drive
See if the box next to Desktop & Documents Folders is checked.
If that option is checked and your library is in either of those locations or a subfolder of them, that can cause this to occur.