I need to upgrade my OS from Catalina to Big Sur.

I have an Early 2013 MacBook Pro Retina 15in. I am trying to follow the instructions found on apple/ support site for installing an older OS as it does not appear that my Mac will be able to upgrade to Monterey OS. The instructions say to download the new OS with Safari. But when I click the link it opens the app store. Then within the app store, when i click the download link I get a message that the file is unavailable. Any ideas? My accounting software won't work with the old OS anymore. I am currently running Catalina 10.15.7 When I check for softwar Updates throught the automatic update app it says i am on the latest version. Thanks!