ksu: 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/ksu - from package: krb5-workstation

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

On Debian possible default full paths are:

/usr/bin/ksu - from package: krb5-user

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

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