can I upgrade to MacOS Catalina on a 2011 MacBook Air?

Hi, welcome to the Apple Users Community.

please see this link for more help

https://support.apple.com/en-us/HT201475

I am pretty sure your older system is not capable of running Catalina, the newer one however according to the link...

Best of luck

Here are the requirements for Catalina:

https://support.apple.com/kb/SP803?locale=en_US