airport-linkmon: 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-linkmon - from package: airport-utils

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

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