I am unable to download Catalina or Big Sur on my MacBook Air. Not visible from App Store or System Preferences. What else can I do?

How old is your Mac? : see Apple icon, about this Mac

Look here if your Mac is compatible with Catalina:

https://support.apple.com/en-us/HT210222

If not, Big Sur, also isn’t an option and SystemPreferences won’t show you an update

Hi,

You can download from following sites.

Big Sur : https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=12

Catalina : https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12