I have a 2009 desktop 21" Mac-can i run os El Capitan?

Hello. Yes, you can run Mac OS X El Capitan on your 2009 iMac. Depending on if your iMac is an Early, Mid, or Late 2009 model, you may also be able to upgrade to macOS 10.13 High Sierra as well. Early and Mid 2009 iMacs cannot upgrade beyond 10.11, but Late 2009 iMacs can upgrade to macOS High Sierra. You can find information about your iMac's specific model by clicking the Apple logo in the top-left of your screen and choosing "About This Mac".

You can get old versions of macOS by clicking the links in this article on your Mac to open the App Store. You can then choose the appropriate version of macOS for your iMac, download the installer, and run it to begin the upgrade process. As always, please be sure to backup your iMac before proceeding with the upgrade.