su: 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:

/bin/su - from package: coreutils
/usr/lib/heimdal/bin/su - from package: heimdal-workstation

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

On Debian possible default full paths are:

/bin/su - from package: login

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

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