Two reasons can drive to this issue in Spark:
Your tracks are protected
Some music plateform (like apple music) are protecting the tracks they are selling with DRM (Digital Rights Management). Spark is not able to read this protection, so he will show the track with the mention "Not readable".
From Spark, when you play specific local music, Spark shows the error message «Not Readable».
In order to protect their music from piracy, some editors encrypt their music with DRM (Digital Rights Management).
This protected music can't be read within Spark.
There is some possibility to remove the protection by contacting the music provider.
Your tracks are hosted in the "Cloud"
When the tracks your are trying to read is not hosted into your local device, Spark won't be able to read it and will display aside the track the mention "Not readable".
To solve this type of behavior, please download locally the track from the application you have purchase the products, and re-open Spark.
For itunes you can find the icons and their definitions in the following page:
Note: If you are using Phantom with BlueTooth, there is no need to use Spark, therefore, you won’t have problems to read encrypted or not stored locally music.