airport-modem: 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/airport-modem - from package: airport-utils

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

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