Error: /usr/local must be writable! (update homebrew)

I ran: brew update in terminal (Mac OS) output: Error: /usr/local must be writable!

Anyway, as you can probably tell I am new to working with this. I searched this expecting it to be a common question but only saw not so helpful answers on github. Any help would be appreciated. *Note my terminal did recently switch to zsh with the last update (not too sure what that is).

Solution 1:

Wound up just reinstalling homebrew with /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)" found at https://brew.sh/ . This cleared everything up and I can download packages. Still not sure why brew update didn't do the trick.