openntpd: 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/sbin/openntpd - from package: openntpd

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

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