That exists in CIFS and NFS SVMs, but not in iSCSI. Volumes in iSCSI SVM's with qtrees cannot be marked offline because of active junction paths, and the popup message says to unmount the volume from the Namespace window to take the volume offline. Without a Namespace menu in an iSCSI SVM, you have to do it via command-line.
::>volume show -vserver vserver_name -volume volume_name -junction
::>volume unmount -vserver vserver_name -volume volume_name