ntpd: 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/ntpd - from package: ntp

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

On Debian possible default full paths are:

/usr/sbin/ntpd - from package: ntp
/usr/sbin/ntpd - from package: openntpd

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

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