mpirun.openmpi: 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 Debian possible default full paths are:

/usr/bin/mpirun.openmpi - from package: openmpi-bin

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

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