I am running macOS High Sierra 10.13.2 on my MacBook Pro. After upgrading the OS earlier today, I also wanted to give the applications an upgrade that I had installed using HomeBrew.
I typed “brew update && brew upgrade” in the terminal and got the following error
Error: /usr/local is not writable. You should change the ownership and permissions of /usr/local back to your user account: sudo chown -R $(whoami) /usr/local
sudo chown -R $(whoami) /usr/local
chown: /usr/local: Operation not permitted
To make a long story short, I ended up with re-installing HomeBrew with
/usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
After the re-install, everything works.