"Install A Clustered Master Server" Greyed Out

I am trying to set up a test NBU 7.6.0.1 master server cluster with VCS but the option to "install a clustered master server" is greyed out.

I have built 2 new servers, installed SFHA / VCS 6.0.1 and configured disks / volumes / disk groups / VVR replication. The VCS installation has not been modified or customised in any way as from my understanding the NBU installation will create its own service group.

I've probably just made a silly mistake somewhere but I can't figure out what piece of the puzzle I have missed - any ideas?