curvetun: 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/curvetun - from package: netsniff-ng

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

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