title: brew update Error: The /usr/local directory is not writable. sudo chown -R $(whoami):admin /usr/local not working
body: <p>I am getting this error:</p>
  
  <pre><code>jaimes-mbp:SMR jaimemontoya$ brew update
  Error: The /usr/local directory is not writable.
  Even if this directory was writable when you installed Homebrew, other
  software may change permissions on this directory. Some versions of the
  "InstantOn" component of Airfoil are known to do this.
  
  You should probably change the ownership and permissions of /usr/local
  back to your user account.
    sudo chown -R $(whoami):admin /usr/local
  jaimes-mbp:SMR jaimemontoya$ brew update
  Error: The /usr/local directory is not writable.
  Even if this directory was writable when you installed Homebrew, other
  software may change permissions on this directory. Some versions of the
  "InstantOn" component of Airfoil are known to do this.
  
  You should probably change the ownership and permissions of /usr/local
  back to your user account.
    sudo chown -R $(whoami):admin /usr/local
  jaimes-mbp:SMR jaimemontoya$ sudo chown -R $(whoami):admin /usr/local
  Password:
  chown: /usr/local: Operation not permitted
  jaimes-mbp:SMR jaimemontoya$ 
  </code></pre>
  
  <p>For some reason, it is not letting me to change the ownership and permissions of <code>/usr/local</code>.</p>