elk_mpich: 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/mpich/bin/elk_mpich - from package: elk-mpich

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

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