sdcc-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 CentOS possible default full paths are:

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

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

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