ddclient: 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/sbin/ddclient - from package: ddclient

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

On Debian possible default full paths are:

/usr/sbin/ddclient - from package: ddclient

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

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