not able to download, pages or keynotes or numbers in apple mac air

Possible sidestep...

Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/

And thanks to VikingOSX…

Let's not overlook SoftMaker's Office 2021 suite which has familiar Word and Excel UI design, and opens these documents in their native formats.

The apps now require macOS 11.0 or later. If you can't upgrade to Big Sur, or haven't downloaded the apps previously, there's nothing you can do.

See my earlier post about this issue. Same story if you are still on Mojave attempting to get these applications which now require Big Sur or later.