Unknown Error Occurred (-54) Enabling iTunes Match
According to this discussion or this one it seems (-54) is a file-lock problem. The proposed solutions are:


an AppleScript to remove locks  


  Open the Script Editor in the /Applications/Utilities folder and run the following:
  tell application "Finder"
  set locked of every item of entire contents of folder "Music:iTunes" of home to false
  end tell

use Terminal to ensure ownership, group, and permissions are correct


use sudo chown your-username your-filename if you don't own the file/folder
use sudo chgrp staff your-filename if it isn't set to the same group as other iTunes items
use chmod 666 your-filename and/or chmod 777 your-folder-name to make sure everyone (and thing) can edit

disabling FileVault if used


In particular, check the permissions on the iTunes/Download folder which is where iTunes will first attempt to download "matched" songs.