xnbd-register: 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 Debian possible default full paths are:

/usr/sbin/xnbd-register - from package: xnbd-common

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

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