First check if the nfs storage is working properly and you are able to mount it on Hypervisor.
mount -t nfs management-server-ip:/nfsshare /mnt
If this is working properly then you can add this nfs share as secondary storage.
Once secondary storage has been added make sure you register system vm template.
Mount the nfs share on management server itself and register the template:
mount -t nfs management-server-ip:/nfsshare /mnt
/usr/share/cloudstack-common/scripts/storage/secondary/cloud-install-sys-tmplt \
-m /mnt/secondary \
-u http://cloudstack.apt-get.eu/systemvm/4.6/systemvm64template-4.6.0-kvm.qcow2.bz2 \
-h kvm -s <optional-management-server-secret-key> -F
Download the template as per your hypervisor and then enable the zone.
After this ssvm should work fine.
I hope this helps.