xnbd-server: command not found

Have You tried accessing the binary with the full path?

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-server - from package: xnbd-server

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