The folder "iTunes" is on a locked disk or you do not have write permissions for this folder.

Problem: iTunes requires write access to the iTunes directory located in your local user's music directory.

MAC OS X
1. Navigate to /Users/home/Music
2. Command+i on the iTunes directory (Opens the directory info)
3. Click the padlock in the bottom right and enter administrator credentials for the local machine...
4. Add your current user to the list of users with access (If it's not currently there)
5. Open the 'Privilege' dropdown for the current user, and select 'Read & Write'.
6. Click the padlock to lock the permissions.
7. Restart iTunes.


Windows
I don't have a Windows machine handy at the moment, but the process will be similar...

1. Navigate to 'My Documents' for the current user.
2. Right click and Properties for the iTunes directory (Either in here, or in the 'My Music' directory.
3. Set write permissions for the logged in user.
4. Restart iTunes.