Cannot update macOS with app store
I have Cataline need to ugrarde to big sur or monterey but app store says update not available
iMac 21.5″, macOS 10.15
MacOS versions are all "put away" so that you do not find and download them by accident.
Provided your Mac (what model year?) is qualified, you can get them by using the Mac App Store special links in this article:
How to download and install macOS - Apple Support
Provided your Mac is already operating properly, downloading using the App Store is the most reliable method, because the first part of the setup takes place under full MacOS, not just the severely limited Recovery software.
Remember that you should have a trusted local disk-based backup available in case something goes wrong. You will be re-writing over 30,000 files.