nbd-client: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/sbin/nbd-client - from package: nbd

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/sbin/nbd-client - from package: nbd-client

You may install the required package with command apt-get install {package}

Error in other languages:
nbd-client: bevel nie gevind nie
nbd-client: comandă negăsită
nbd-client: comando não encontrado
nbd-client: commande introuvable
nbd-client: command not found
nbd-client: käsku ei ole
nbd-client: không tìm thấy lệnh
nbd-client: komanda nerasta
nbd-client: Kommando nicht gefunden.
nbd-client: kommandot finns inte
nbd-client: komut yok
nbd-client: nie znaleziono polecenia
nbd-client: níor aimsíodh an t-ordú
nbd-client: no se encontró la orden
nbd-client: no s'ha trobat l'ordre
nbd-client: opdracht niet gevonden
nbd-client: parancs nem található
nbd-client: perintah tidak ditemukan
nbd-client: príkaz nenájdený
nbd-client: příkaz nenalezen
nbd-client: команда не найдена
nbd-client: командата не е открита
nbd-client: 命令找不到
nbd-client: コマンドが見つかりません