Proxmox need access using SSH to target iSCSI device (i.e. Zyxel NAS326).
It is trying to execute following command:
/usr/bin/ssh -o 'BatchMode=yes' -i /etc/pve/priv/zfs/192.168.28.150_id_rsa root@192.168.28.150 zfs list -o name,volsize,origin,type,refquota -t volume,filesystem -Hr
To give access to target SSH, create new SSH access key and authorize it on target machine:
# run on Proxmox node: mkdir /etc/pve/priv/zfs ssh-keygen -f /etc/pve/priv/zfs/192.168.28.150_id_rsa ssh-copy-id -i /etc/pve/priv/zfs/192.168.28.150_id_rsa.pub root@192.168.28.150 # login once to target from each proxmox node, to accept key: ssh -i /etc/pve/priv/zfs/192.168.28.150_id_rsa root@192.168.28.150