airmon-ng: 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/airmon-ng - from package: aircrack-ng

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

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