Hi Frederick,
The iTunes library file may not be specified properly. To designate the proper iTunes Library xml file, go to [Preferences] -> [Advanced] -> [Database] and select the xml file for the iTunes Library.
With rekordbox settings files transferred from a different computer, the iTunes Library file may be changed from iTunesMusicLibrary.xml to iTunesLibrary.xml. If this is the case with your file, change the above setting to iTunesLibrary.xml.
iTunes Library may not be always created when using iTunes12.2 and later.
From iTunes -> Preferences -> Advanced, tick the box of “Share iTunes Library XML with other applications” to enable this function.
Then, please reboot both iTunes and rekordbox after shutting them down.