hi i am not able to download itune on my macbook air MacBook Pro (Retina, 15-inch, Mid 2015)

See: What happened to iTunes? - Apple Support

You are running Big Sur. Before that came Catalina. "With macOS Catalina [and later], your iTunes media library is now available in the Apple Music app, Apple TV app, Apple Books app, and Apple Podcasts app. And Finder is where you can manage and sync content on your iPhone, iPad, and iPod touch." The various functions of iTunes were split up into different applications.

Note that "iTunes" refers to an application. It does not mean the music (which is called music), or movies (called movies) or your Apple account, or the Apple Music service, etc.