Have IMac 21.5 inch late 2013 compatible to OS 11, Big Sur?

It can't be upgraded past Catalina.

(207033

To be clear, software update generally means to install software to update the macOS currently running on the Mac; update Catalina 10.15.1 to Catalina 10.15.7, for example.

To update your current macOS, click the Apple  icon > About This Mac and then click Software Updates.... If there are any updates available for your Mac, they will be listed and you can apply them when you like. Updates can also be applied automatically if you set the proper allowances in System Preferences > Software Update.

A software upgrade is usually a generational step up to a newer macOS; upgrade from 10.13 High Sierra to 10.14 Mojave, for example, or from 10.15 Catalina to macOS 11 Big Sur. Software upgrades do not happen automatically. They must be initiated by the user.