The volume needs to be mounted in order for iTunes to find the Library file.
For example mine is located here:
/ Volumes /Public/Shared Music/iTunes/iTunes Media
If it can’t find the volume it will throw the error.
One way round it is to add the volume to your user login items.
First of all go to Finder Preferences and under general make sure ‘External disks’ is checked under ‘Show these items on the desktop’.
Open the share where your iTunes library is located and the volume will appear on the desktop.
Go to System Preferences > Users & Groups
Select your user name and select the ‘Login Items’ tab.
Drag the volume from the desktop into the login items lists and check the box by the volume name.
To test it works, log off and on again. You should see the volume appear on the desktop.
Not sure how this will work if you have restarted your mac and/or the MBL has to wake up? You make need to write an apple script, saved as an app and placed in the login items, that mounts the volume using a ‘try’ loop. So it keeps trying until the MBL is available.