This is related to sandboxing on MacOS when your music is stored in a different place than the Music folder. The solution is to choose File -> Allow Access to Folder... and then select the folder at the top of where your music is stored. The default folder
when the dialog comes up is the root folder for your Mac, if you choose it, this will also solve the problem.
By selecting a folder, you are giving the Windows Phone app permission to access files in that folder and this is necessary for it to sync the files to your phone. Note that this could also happen with your photos from iPhoto or Aperture if they are stored
outside of the Pictures folder.
You should only need to select the folder once. The Windows Phone app will remember this location and future syncs will continue to work.
Jeff Pearce Senior SDE - Microsoft Windows Phone Companion Experiences
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