rhnreg_ks: 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/sbin/rhnreg_ks - from package: rhn-client-tools

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

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