Assuming you're using High Sierra (OS version 10.13+), Apple has introduced some protection around the installation of drivers. This includes Soundflower.
Unfortunately, installers need to be modified to return the a more sensible message. This is never going to happen for Soundflower (since it's old and AFAIK not being maintained).
However; the process is simple when you know you have to do it:
- Go through the install process.
- The installer will FAIL at the end (it hasn't actually, continue on)
- Go to System Preferences | Security & Privacy | General
- Click 'Allow' and Allow the item "MATT INGALLS"