hydra_pmi_proxy: 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/hydra_pmi_proxy - from package: mpich
/usr/lib/mpich/bin/hydra_pmi_proxy - from package: mpich
/usr/lib64/mvapich2-psm/bin/hydra_pmi_proxy - from package: mvapich2-psm
/usr/lib64/mvapich2/bin/hydra_pmi_proxy - from package: mvapich2
/usr/lib/mvapich2/bin/hydra_pmi_proxy - from package: mvapich2

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

On Debian possible default full paths are:

/usr/bin/hydra_pmi_proxy - from package: mpich

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

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