Aaaah, the next version of Mac OS X. So tempting. The follow-up to "El Capitan" is called "Sierra" (v10.12.0). See the Wiki [HERE]. If you value your ability to operate Media Composer...
WARNING #1: DO NOT update your OS yet. Not until Avid makes a version of MC that can run on it. No version of MC that you have running right now will operate on Sierra... Ever. Not MC8.x, MC7.x, MC6.5.x or whatever. You will need to download a new version (or patch upgrade) of Media Composer, qualified for Sierra, once Avid makes it.
WARNING #2: Sierra will be the first version since OS X Mountain Lion, released in 2012, that DOES NOT RUN on all computers that the previous version supported. An article going into some detail is [HERE].
WARNING #3: Even though you shouldn't install Sierra, you SHOULD download it for later. (Just don't install it.) Otherwise if you need the first release (10.12.0), you'll have it handy. (Thanks to the ever-awake Shane Ross for the reminder on this one!)
So... I know I sound like a broken record here, but Avid's engineers and app designers are aware of Sierra. They have been working with Apple on a beta version of it, so that they can bring everyone a next version of Media Composer that will run on it. But at this very moment while they are doing so, Apple is also making last-minute changes to Sierra for its release... and those changes may indeed affect all of the work Avid has done. They need to test their next version of MC on the SAME version available to the public, starting tomorrow.
So be smart and WAIT. Don't upgrade, and while you're at it, turn off auto-updates immediately.