Sometimes there can be a problem with having the operating system accept the Soundflower kernel extension (kext) if it was not approved suring the first installation.

The problem can be solved by going to System Preferences -> Security and Privacy. At the bottom of the window, there should be a note about "System software" being blocked form loading. Please see attached image. If you see the Soundflower kext here, click "Allow".

Then Soundflower should be allowed to load. Maybe you have to reboot the machine once in order to make it active.

If you don't see any extensions listed in this place, you might have to reinstall once more, and then go to Security and Privacy again and click allow. That is because there is a limited time window (30 minutes) for allowing the kext after a load attempt was made.

 

There is some more information about the issue in Apple's Documentation.