rdrand-gen: 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/bin/rdrand-gen - from package: RdRand

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

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