ntp-keygen: 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/ntp-keygen - from package: ntp

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

On Debian possible default full paths are:

/usr/sbin/ntp-keygen - from package: ntp

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

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