My goal is to add additional network to vcenter appliance 6.0 and i am used to using yast2.
it works in 5.5 but I am unable to run that command in V6.0.
I'd say the command was removed, as VMware sees their appliances as black boxes, that shoudln't be modified by the user.
So if this is your production vCenter, I woudln't add another network, as you might get trouble in your next support call with VMware.
try to run it as root:
sudo su -
and then yast
what about if you try first to enable the shell?