Unable to run Postgres after Installation on OSX - El Capitan
First, I would advise running brew doctor to make sure your homebrew installation is still intact.  It will instruct you on changes that need to be made after your upgrade.

If you are still having trouble and have database backups or nothing critical that you believe could get broken, I believe an easy fix would be to run something like

brew uninstall postgres &amp;&amp; brew install postgres


By default, homebrew recognizes your OS version, and will reinstall from a "bottle" specifically-intended for El Capitan, which should fix your particular issue.

You could also try brew upgrade postgres (or whatever the equivalent command is).