puttygen: 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/puttygen - from package: putty

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

On Debian possible default full paths are:

/usr/bin/puttygen - from package: putty-tools

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

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