FYI you only have a title to your comment and no description about your issue. Please elaborate on what your problem is and information about your project.
I don't know why I can't post a pic here to show you the problem, it's just when I drag the audio fade to the music, a forbidden sign shows up, can not apply the effect on the audio track.
@Michael19872018 I believe that audio fade is a transition, not an effect per se, so in order to apply it you need to drag it to one of the ends of the clip. If you are trying to just fade out the volume of the track, you should be able to keyframe the levels instead.