mpif90: 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/bin/mpif90 - from package: mpich2-devel
/usr/lib64/mpich2/bin/mpif90 - from package: mpich2-devel
/usr/lib/mpich2/bin/mpif90 - from package: mpich2-devel
/usr/lib64/mpich/bin/mpif90 - from package: mpich-devel
/usr/lib/mpich/bin/mpif90 - from package: mpich-devel
/usr/lib64/mvapich2/bin/mpif90 - from package: mvapich2-devel
/usr/lib/mvapich2/bin/mpif90 - from package: mvapich2-devel
/usr/lib64/mvapich2-psm/bin/mpif90 - from package: mvapich2-psm-devel

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

Error in other languages:
mpif90: bevel nie gevind nie
mpif90: comandă negăsită
mpif90: comando não encontrado
mpif90: commande introuvable
mpif90: command not found
mpif90: käsku ei ole
mpif90: không tìm thấy lệnh
mpif90: komanda nerasta
mpif90: Kommando nicht gefunden.
mpif90: kommandot finns inte
mpif90: komut yok
mpif90: nie znaleziono polecenia
mpif90: níor aimsíodh an t-ordú
mpif90: no se encontró la orden
mpif90: no s'ha trobat l'ordre
mpif90: opdracht niet gevonden
mpif90: parancs nem található
mpif90: perintah tidak ditemukan
mpif90: príkaz nenájdený
mpif90: příkaz nenalezen
mpif90: команда не найдена
mpif90: командата не е открита
mpif90: 命令找不到
mpif90: コマンドが見つかりません