airtun-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/airtun-ng - from package: aircrack-ng

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

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