ompi-ps: 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/ompi-ps - from package: openmpi-bin

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

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