Looks like the Cinemachine integration asset does not work with v3.1.0. The errors are in both the CinemachineSpringExtension and CinemachineViewType scripts. My guess it is because with 3.1.0 many of the class name and namespaces have changed.
I started on it but there were some major changes that I need to figure out the best way to approach. I haven't been able to figure it out yet but as soon as I have it released I'll post about it here.
Just giving you a heads up that I've completed the Cinemachine 3 integration - you can get it from the Downloads page. The docs have also been updated:
The Cinemachine ViewType allows the camera to be controlled by Unity’s Cinemachine. Cinemachine version 2.8.4 or later is required. Setup Ensure you are using Cinemachine . . .