Everyone loves Brew, except when Brew doesn’t Brew.
Error: /usr/local must be writable!
This is caused by ownership and permission issues of the /usr/local folder and the currently controlled account.
You can sudo the update however Brew may cry and refuse without changing the ownership of the executable to root.
For a quick fix, run the following:
sudo chgrp -R admin /usr/local
sudo chmod -R g+w /usr/local
Sometimes things go wrong inside the world of /usr/local and certain items lose their ability to be written to by all admin users. These commands effectively remind everyone who’s boss.