mpirun.py: 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/mpirun.py - from package: mpich2

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

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