nbd-server: 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/bin/nbd-server - from package: nbd

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

On Debian possible default full paths are:

/bin/nbd-server - from package: nbd-server

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

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