What command did you miss? I'm having the same error ("Error starting domain: Requested operation is not valid: network 'Whonix' is not active").
EDIT: I figured it out. I found the following steps in the original installation:
virsh -c qemu:///system net-autostart default
virsh -c qemu:///system net-start default
Apparently, you also need:
virsh -c qemu:///system net-autostart Whonix
virsh -c qemu:///system net-start Whonix
I tried different things, but that seemed to get it working.