music app disappeared and wont download

Music is a built-in app that you can delete:

Delete built-in Apple apps from the Home Screen on your iOS 14 or later or iPadOS device or Apple Watch - Apple Support

If you can't redownload it from the App Store, then perhaps it is on another screen or buried in a folder. See if you can find it in your App Library and move it to wherever you want:

Organize the Home Screen and App Library on your iPhone - Apple Support