sdiff: 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/sdiff - from package: diffutils

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

On Debian possible default full paths are:

/usr/bin/sdiff - from package: diffutils

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

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