ddns3-client: 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/ddns3-client - from package: ddns3-client

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

Error in other languages:
ddns3-client: bevel nie gevind nie
ddns3-client: comandă negăsită
ddns3-client: comando não encontrado
ddns3-client: commande introuvable
ddns3-client: command not found
ddns3-client: käsku ei ole
ddns3-client: không tìm thấy lệnh
ddns3-client: komanda nerasta
ddns3-client: Kommando nicht gefunden.
ddns3-client: kommandot finns inte
ddns3-client: komut yok
ddns3-client: nie znaleziono polecenia
ddns3-client: níor aimsíodh an t-ordú
ddns3-client: no se encontró la orden
ddns3-client: no s'ha trobat l'ordre
ddns3-client: opdracht niet gevonden
ddns3-client: parancs nem található
ddns3-client: perintah tidak ditemukan
ddns3-client: príkaz nenájdený
ddns3-client: příkaz nenalezen
ddns3-client: команда не найдена
ddns3-client: командата не е открита
ddns3-client: 命令找不到
ddns3-client: コマンドが見つかりません