dis51: 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/dis51 - from package: dis51

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

Error in other languages:
dis51: bevel nie gevind nie
dis51: comandă negăsită
dis51: comando não encontrado
dis51: commande introuvable
dis51: command not found
dis51: käsku ei ole
dis51: không tìm thấy lệnh
dis51: komanda nerasta
dis51: Kommando nicht gefunden.
dis51: kommandot finns inte
dis51: komut yok
dis51: nie znaleziono polecenia
dis51: níor aimsíodh an t-ordú
dis51: no se encontró la orden
dis51: no s'ha trobat l'ordre
dis51: opdracht niet gevonden
dis51: parancs nem található
dis51: perintah tidak ditemukan
dis51: príkaz nenájdený
dis51: příkaz nenalezen
dis51: команда не найдена
dis51: командата не е открита
dis51: 命令找不到
dis51: コマンドが見つかりません