rhn_register: 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/bin/rhn_register - from package: rhn-client-tools
/usr/sbin/rhn_register - from package: rhn-client-tools

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

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