try running it at the cluster level, not the "vserver cifs" level. If you are running it while ssh'ed into the vserver level, you will likely need to just leave the "-vserver vservername" section out, because you are already in the vserver.
cluster::> cifs security modify -vserver <vserver> -smb1-enabled-for-dc-connections false -smb2-enabled-for-dc-connections true
or
vserver::> cifs security modify -smb1-enabled-for-dc-connections false -smb2-enabled-for-dc-connections true