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

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

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