For some reason some songs in your iTunes library are DRM protected.
If you enable the Kind column on your iTunes Library you see the file type for every song. In the example above Nimbus 2000 is a Protected AAC audio file.
Unfortunately Apple does not allow access to these files from within another app, so it is impossible to import this type of songs into Mapping Tonal Harmony Pro
If you try to drag & drop one of these files Mapping Tonal Harmony will show a red background in the drag & drop area