airolib-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/airolib-ng - from package: aircrack-ng

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

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