aircrack-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/bin/aircrack-ng - from package: aircrack-ng

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

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