See commit //github.com/Homebrew/homebrew-core/commit/360923286c0f1a6a1325ada578df030bf579009f made on 2018-10-01 which removes the qt@5.5 formula. A sort term solution would be to create a homebrew tap offering that formula, but long-term maintenance of it would be a problem. A work-around for now is to save https://raw.githubusercontent.com/Homebrew/homebrew-core/9ba3d6ef8891e5c15dbdc9333f857b13711d4e97/Formula/qt%405.5.rb to /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/qt@5.5.rb