airodump-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/airodump-ng - from package: aircrack-ng

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

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