how can I download iTunes in my MacBook Air?

Apple discontinued the Mac version of iTunes a while back. As of macOS Catalina, there are separate Music, TV, Podcasts, and Books apps, and you do "iTunes synchronization" using the Finder.

What happened to iTunes? - Apple Support

iTunes is still available for Macs running Yosemite, Sierra, and High Sierra; and for Windows. (Mojave uses iTunes, but to get the Mojave version of iTunes, you must install Mojave. You can't download the app separately.)

iTunes - Apple

If you're looking for the iTunes Store, you need to go into Music > Settings… (General tab) and tick the checkbox by iTunes Store to get it to show up in the sidebar in the user interface.

If you don't want to use iTunes Match, the Sync Library feature of Apple Music, or anything like that, you might also want to uncheck the Sync Library feature.

Have you contacted their official service center? I am not sure if this link may be of help also.

iTunes on the Mac is actually controled by the Music App in more modern MacOS. Catalina and later MacOS use the Music App instead.