I’m unable to install the Big Sur on my 2014 Mac air with 512 K memory.

Big Sur runs fine on that model Mac.

If you purchased it "used" then its seller must comply with Apple's instructions prior to selling it.

What to do before you sell, give away, or trade in your Mac - Apple Support

Those steps are not optional.

Make sure they do NOT install the OS. They can download it and then quit the installer and shut down (see the instructions towards the bottom of the page). That way it is not assigned to an Apple ID and you can install and set it up as your own.

to make sure your computer is compatible, check this website. https://support.apple.com/en-us/HT211238