plainrsa-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/sbin/plainrsa-gen - from package: ipsec-tools

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

On Debian possible default full paths are:

/usr/sbin/plainrsa-gen - from package: racoon

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

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