pypop: 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/pypop - from package: pypop

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

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