pen: 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/pen - from package: pen

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

On Debian possible default full paths are:

/usr/bin/pen - from package: pen

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

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