The problem with using FreeNAS for a Proxmox is that the default FreeNAS NFS configuration is to map root to nobody. This makes it so that you can not use chmod or chown. Proxmox needs to use both chmod and chown. Proxmox wants to be root of the NFS
To fix this you change the Maproot User and Maproot Group in the FreeNAS NFS configure to root and wheel respectively.
PS: If you incorrectly configured the NFS and have a CT that can not be deleted you can remove it by deleting its configuration file in /etc/pve/openvz/
Update: showmount -e nsfServer is helpful for figuring out if the NFS server is working.
To fix this you change the Maproot User and Maproot Group in the FreeNAS NFS configure to root and wheel respectively.
PS: If you incorrectly configured the NFS and have a CT that can not be deleted you can remove it by deleting its configuration file in /etc/pve/openvz/
Update: showmount -e nsfServer is helpful for figuring out if the NFS server is working.