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

Doing a

sudo chown -R $(whoami) /usr/local

results in

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.