dd: 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:

/bin/dd - from package: coreutils

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

On Debian possible default full paths are:

/bin/dd - from package: coreutils
/usr/lib/plan9/bin/dd - from package: 9base
/usr/lib/klibc/bin/dd - from package: klibc-utils

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

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