Files Downloaded from Apple music are often in an m4p format.  These files are encrypted by Digital Rights Management (DRM).  These files cannot be opened directly by Song Surgeon.  The work around, which will break the encryption is to burn the files to a CD and then rip them back off again.  A second option is to concert the file to an mp3 format.   Both of these approaches will strip the DRM protection from the files and allow you to open them.

Here is a link which shows you how to convert your Apple Music files to mp3's.

https://www.tunefab.com/tutorials/convert-apple-music-to-mp3.html

Once you have convert your m4p file to an mp3, you should then be able to open this new mp3 file in Song Surgeon.