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

  1. Run the Replay Video Capture 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 Video Capture”
  3. Re-run the Replay Video Capture installer again and finish the installation

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

https://www.fluxforge.com/blog/soundflower-os-x-10.11-10.12-macOS-sierra/