Hello Raghu
You are correct. It seems that this particular switch version is behaving strangely. It has the ipv6 nd command available, but if you add anything after that, it says “unrecognized command.”
SW1(config)#inter gig 0/1
SW1(config-if)#ipv6 n?
nd next-hop-self
SW1(config-if)#ipv6 nd ?
% Unrecognized command
SW1(config-if)#ipv6 nd
Based on this Cisco learning network thread, others have faced similar situations. Still, others are able to implement RA Guard on this image. According to this documentation, it seems that IPv6 is not supported by this image (even though some of the commands appear!!):
One option is to use the NS-OX v which supports it. The commands are almost the same and the logic is the same. Alternatively, you can try downgrading to the previous IOSvL2 version as some threads state that this may resolve the issue.
Let us know how you get along!
I hope this has been helpful!
Laz