The app might require an update. Check if there are any available updates for the app you're using to create the story. Out - of - date apps can sometimes have issues with adding music.
It could be that the platform you're using doesn't support adding music for certain reasons. Maybe it's a licensing issue or a feature that's not available for your account type.
Maybe there's a technical glitch or a setting that's not enabled properly. Check your app settings and see if there are any permissions related to adding music that need to be granted.