al: 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 CentOS possible default full paths are:

/usr/bin/al - from package: mono-devel

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/al - from package: mono-devel

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

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