network-admin: 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/network-admin - from package: gnome-system-tools

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

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