I just used homebrew to install my postgres, and I am unable to start it after attempting to start it with the suggested command from postgres/brew.
Can someone please advise?
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 && brew install postgres
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).