dyndns: 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/dyndns - from package: dyndns

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

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