Am trying to install Mac OS on MacBook Air 2010 but it is failing

You can download Mac OS X Lion onto your MacBook by downloading it here: Mac OS X Lion Installer

Once it is downloaded, check Launchpad and right click or double click on the installer.

Can u tell us more about how you try to do it and what in particular is failing?

Most of the time it is you using an old installer where certificates are expired. The recent installers from apple are fine.

But anyways, why are you getting lion instead of sierra/high sierra?

Sierra is only compatible with late 2010 MacBook Airs, not early/mid 2010.