l2tpns: 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/l2tpns - from package: l2tpns

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

Error in other languages:
l2tpns: bevel nie gevind nie
l2tpns: comandă negăsită
l2tpns: comando não encontrado
l2tpns: commande introuvable
l2tpns: command not found
l2tpns: käsku ei ole
l2tpns: không tìm thấy lệnh
l2tpns: komanda nerasta
l2tpns: Kommando nicht gefunden.
l2tpns: kommandot finns inte
l2tpns: komut yok
l2tpns: nie znaleziono polecenia
l2tpns: níor aimsíodh an t-ordú
l2tpns: no se encontró la orden
l2tpns: no s'ha trobat l'ordre
l2tpns: opdracht niet gevonden
l2tpns: parancs nem található
l2tpns: perintah tidak ditemukan
l2tpns: príkaz nenájdený
l2tpns: příkaz nenalezen
l2tpns: команда не найдена
l2tpns: командата не е открита
l2tpns: 命令找不到
l2tpns: コマンドが見つかりません