mpiexec.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/mpiexec.py - from package: mpich2

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

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