It might be due to regional restrictions. Some music is only available in certain countries or regions. Try changing your VPN settings if you are using one to see if that helps. Another thing is that your account may be restricted in some way. Contact Instagram support to find out if there are any issues with your account regarding adding music to stories.
There could be several reasons. First, make sure you have the latest version of Instagram. Sometimes an old version may have glitches with adding music. Also, check your app permissions. If Instagram doesn't have access to your device's music library, it won't be able to let you add music to your story.