I see you're running Media Center 18 for Mac. Are you licensed for that version? If not, and you're doing a trial, you should definitely upgrade to Media Center 20; it's a much newer version.
I don't have any experience with MC before 20, so I'm not sure I can help. You could try to find the current library and remove it completely in Finder. I'm guessing it's in:
~/Library/Application Support/JRiver/Media Center 18/Libary/
(Tip: To find the user Library directory in Finder, use "go to folder" under the Go menu. (hot key is <shift><Command>g), then type in ~Library and Finder will take you to your Library directory.)
I'd guess that if you deleted the Media Center 18/Library directory, then started up Media Center, it would probably create a new Library. Not sure though; just guessing. Try it at your own risk.
Brian.