I have a macbook air 2014 and it says I can't download it, why?

Because you need a 2015 or later MacBook Air to install Monterey.

You can, however, upgrade to Big Sur if you have the full 8GB RAM in that 2014 Air and a 256GB drive. You will need 46GB of free drive storage if currently running macOS Sierra (10.12.6) or earlier, and 26GB of free storage if upgrading from newer operating systems. Do not attempt to install Big Sur on a 4GB Ram Air with any sized drive, as it will be RAM deficient with horrible performance.

See How to get old versions of macOS - Apple Support. Make a Time Machine backup of your current operating system and do not reuse that TM drive on the upgraded operating system. Use a new TM drive formatted APFS if on Catalina or Big Sur. Also, realize any 32-bit applications, or printer/scanner drivers that you now use on Mojave or earlier are not supported on Catalina and later operating systems. Look before you upgrade.

you cannot download monterey because your MBA is not compatible with Monterey. you need to use a compatible mac to download and / or install monterey.