There could be several reasons. First, check if your device has the necessary permissions for Instagram to access music. Also, make sure you are in a region where Instagram allows music on stories. If you've recently updated your device or the app, sometimes there can be glitches. Try restarting your device and then opening the app again.
It could be that you don't have the proper permissions or your music file format isn't compatible. Sometimes, the platform's rules might also prevent it.