I see no information about an error 54 but there is an 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.
Try quitting iTunes, then deleting the .xml (not .itl !) version of the library file in the iTunes folder (iTunes will generate a new one).
iTunes Library cannot be saved (Error -54) - https://discussions.apple.com/thread/1912814 - various things to try.
Unknown error (-54) while syncing ipod - https://discussions.apple.com/thread/1082953 - problem was locked files
Why does iTunes keep showing a -54 error when I sync my iPad? - https://discussions.apple.com/thread/3727114
Error -54 possibly related to Touch Copy - https://discussions.apple.com/thread/3727114 - cleared by deleting preferences on iPad.
iTunes: Missing folder or incorrect permissions may prevent authorization - http://support.apple.com/kb/ts1277
Troubleshooting permissions issues in Mac OS X - http://support.apple.com/kb/HT2963