This issue has cropped up on some High Sierra installations that have never had Replay Music installed before. Here's the workaround:

  1. Run the Replay Music installer again and perform an install
  2. When the installer tells you that the installation failed, go to System Preferences > Security & Privacy > General. Then click on “Allow” next to “Replay Music”
  3. Re-run the Replay Music installer again and finish the installation

If you do not see Replay Music in the System Preferences > Security & Privacy > General immediately after a failed install then please install Soundflower directly from this page: