aes2501: 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/aes2501 - from package: aes2501-wy

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

Error in other languages:
aes2501: bevel nie gevind nie
aes2501: comandă negăsită
aes2501: comando não encontrado
aes2501: commande introuvable
aes2501: command not found
aes2501: käsku ei ole
aes2501: không tìm thấy lệnh
aes2501: komanda nerasta
aes2501: Kommando nicht gefunden.
aes2501: kommandot finns inte
aes2501: komut yok
aes2501: nie znaleziono polecenia
aes2501: níor aimsíodh an t-ordú
aes2501: no se encontró la orden
aes2501: no s'ha trobat l'ordre
aes2501: opdracht niet gevonden
aes2501: parancs nem található
aes2501: perintah tidak ditemukan
aes2501: príkaz nenájdený
aes2501: příkaz nenalezen
aes2501: команда не найдена
aes2501: командата не е открита
aes2501: 命令找不到
aes2501: コマンドが見つかりません