dd_rhelp: 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/dd_rhelp - from package: dd_rescue

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

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