This may be old news, but I just resolved the same issue.
I went into iTunes preferences and checked the iTunes Media Folder Location. It was blank. I had to reselect it. (In my case, it was on an external drive.) Not exactly sure how that happened, and I'd just imported a disc an hour earlier, but it resolved my issue.
I also found this resolution on another post about the same error: "Check permissions on your iTunes Music Folder. Navigate to it through Finder, Get Info and make sure permissions are set to read and write. Then click the Gear symbol and Apply to All Enclosed Folders."
Good luck!