pyrsa-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/bin/pyrsa-keygen - from package: python2-rsa

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

On Debian possible default full paths are:

/usr/bin/pyrsa-keygen - from package: python-rsa

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

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