== Error -54 ==
From an OS9 reference, error -54 is a software lock on a file or a permissions error. (Some of the old codes still apply to OSX although there is no formal list of OSX error codes.)
First, try simply restarting your computer.
iTunes Library cannot be saved (Error -54) - https://discussions.apple.com/thread/1912814
iTunes: Advanced iTunes Store troubleshooting - http://support.apple.com/kb/TS3297 > Specific Conditions and Alert Messages: (Mac OS X / Windows) - includes list of iTunes error codes