elk_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 CentOS possible default full paths are:

/usr/lib64/openmpi/bin/elk_openmpi - from package: elk-openmpi

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

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