Problem trying to install iftop via Homebrew
You can remedy things in your current shell by doing:

mkdir -p /usr/local/sbin
export PATH=${PATH}:/usr/local/sbin
brew link iftop


That'll get you past the warnings and let Homebrew install the iftop package. If the iftop package is installing things in to /usr/local/sbin that you're looking to run, you'll need to ensure this is on your $PATH when you open a shell. To do this, edit ~/.bash_profile and add the line:

export PATH=${PATH}:/usr/local/sbin


To the end of the file to prepend /usr/local/sbin to each new shell you open.