s51: 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/s51 - from package: cc1111
/usr/bin/s51 - from package: sdcc-ucsim

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

Error in other languages:
s51: bevel nie gevind nie
s51: comandă negăsită
s51: comando não encontrado
s51: commande introuvable
s51: command not found
s51: käsku ei ole
s51: không tìm thấy lệnh
s51: komanda nerasta
s51: Kommando nicht gefunden.
s51: kommandot finns inte
s51: komut yok
s51: nie znaleziono polecenia
s51: níor aimsíodh an t-ordú
s51: no se encontró la orden
s51: no s'ha trobat l'ordre
s51: opdracht niet gevonden
s51: parancs nem található
s51: perintah tidak ditemukan
s51: príkaz nenájdený
s51: příkaz nenalezen
s51: команда не найдена
s51: командата не е открита
s51: 命令找不到
s51: コマンドが見つかりません