enTrainer supports most mp3, m4a, AIFF and WAV formats. It does not support the the older iTunes Store format that has DRM (Digital Rights Management, aka FairPlay “Protected AAC audio file”). Fortunately, Apple stopped using this format several years ago and songs purchased on iTunes are fully compatible now. 

Should you have older Apple Protected AAC audio files in your library, Apple has an upgrade program called “Music Match”, which will replace these files with higher bitrate and unprotected versions.

Otherwise, if these files are in your playlist, enTrainer will simply skip over them.