After uploading a track via rekordbox, I can’t play the track with the iTunes/Apple Music app.

If [Move] is selected when uploading tracks, the tracks will move from the original folder to the Dropbox folder.

As a result, the tracks in the folder that the iTunes/Apple Music app refers to are deleted and can't be played.

To upload a track and keep it in the folder that the iTunes/Apple Music app refers to, select [Copy]. You can switch this setting in [Preferences] > [Advanced] > [Library Sync].

If you've already moved a track to a Dropbox folder, you can return it to the previous folder by selecting [Cloud Library Sync] > [Move to local storage] in the context menu.