a2def: 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/a2def - from package: alliance

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

Error in other languages:
a2def: bevel nie gevind nie
a2def: comandă negăsită
a2def: comando não encontrado
a2def: commande introuvable
a2def: command not found
a2def: käsku ei ole
a2def: không tìm thấy lệnh
a2def: komanda nerasta
a2def: Kommando nicht gefunden.
a2def: kommandot finns inte
a2def: komut yok
a2def: nie znaleziono polecenia
a2def: níor aimsíodh an t-ordú
a2def: no se encontró la orden
a2def: no s'ha trobat l'ordre
a2def: opdracht niet gevonden
a2def: parancs nem található
a2def: perintah tidak ditemukan
a2def: príkaz nenájdený
a2def: příkaz nenalezen
a2def: команда не найдена
a2def: командата не е открита
a2def: 命令找不到
a2def: コマンドが見つかりません