I'm trying to add songs and I'm getting an error message that the file is not supported. The file is an MPEG-4 downloaded from Apple. I've used the exact same files in the past and never had issues. Did something change for Photostory Deluxe 2024?
Apple Music files are "protected" by a Digital Rights Management system in order to prevent them being used or played other than by iTunes, or similar. This has always been the case but it seems that, previously, you have been able to use such files in PS.
It is possible that changes have been made to the internal working of PS that now prevent this happening, perhaps at the insistence of Apple. You would need to ask Magix about this by emailing infoservice@magix.net.
You say the files are MPEG-4, which implies they are video files. Is this correct? I would expect audio-only files to be m4a. It is possible to (perhaps unofficially) convert such files to a more "standard" format but this is really something you would need to carefully research for yourself.