rhn_check: 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/rhn_check - from package: rhn-client-tools

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

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