As Apple has released El Capitan this week, I strongly recommend all Mac users download the OS from the App Store now, but DO NOT INSTALL IT. After the download is complete, the installer will launch which you can force quit. You'll then find an application called "Install OS X El Capitan" in your Applications folder. Save it for future use.
No Avid products support OS X 10.11 at this time, so please do not install it unless you want to beta test for yourself.
With the base version in hand, you will be able to download any Combo update to install an Avid-qualified OS. Once 10.11.1 comes out, that will be the only version available from the App Store.