The volume-create API is a Vserver API in Clustered ONTAP. You connect to a Vserver and then run the volume-create API to create a volume on that Vserver. Or, alternatively, you can connect to the Administrative Vserver on the Cluster Management interface and use Vserver tunneling (NaServer.setVserver()) to tunnel the volume-create API to your desired Vserver.
You might find the sample code included in the SDK helpful. It has an example of creating volumes using either method described above.