Why don't I see iTunes Store on my iTunes page?

See the information in the iTunes Store section of Add and download music from Apple Music or buy music from the iTunes Store - Apple Support

including:

If you don't see the iTunes Store on your Mac

1. 1. Open the Apple Music app.
   2. In the menu bar, choose Music > Preferences.
   3. Go to the General tab and select iTunes Store.
   4. Click OK.

and

On your Mac using macOS Catalina

1. 1. Open the Apple Music app.
   2. Use the Search bar to find the song or album that you want.
   3. Click the iTunes Store button below the volume controls. Don't see the iTunes Store?
   4. 
   5. Click the price to buy the song or album.
   6. The music appears in your library. To download music so you can listen to it offline, find the song or album in your library, then tap or click the download button .