mpiexec.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 Debian possible default full paths are:

/usr/bin/mpiexec.mpich - from package: mpich

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

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