ns: 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/bin/ns - from package: ns2

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

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