Isso pode ser fechado, pois parece que o problema foi corrigido durante a noite.
$ brew install parallel
==> Downloading https://homebrew.bintray.com/bottles/parallel-20180122.high_sierra.bottle.tar.gz
Already downloaded: /Users/deant/Library/Caches/Homebrew/parallel-20180122.high_sierra.bottle.tar.gz
==> Pouring parallel-20180122.high_sierra.bottle.tar.gz