It could be that your internet connection is unstable. Weak or intermittent connectivity can stop you from posting on your Instagram story. Also, check if you've violated any of Instagram's community guidelines, as that might restrict your posting ability.
It might be because your account has some sort of violation or you haven't updated the app to the latest version. Try restarting your device and reinstalling the Instagram app to see if that helps.
There could be several reasons. Maybe the file format of the music you're trying to add isn't compatible. Or it might be that you haven't enabled the right settings or obtained the necessary permissions to add music to your story.
Perhaps there's a copyright issue with the music you're trying to use. Or it might be that your account doesn't have the necessary permissions. Try using different, royalty-free music and see if that works.
It could be that the file format of your story is not supported. Make sure it's in the correct format as specified by the upload requirements. Also, check if the size of your story exceeds the allowed limit.