mpirun.mpich: command not found

Have You tried accessing the binary with the full path?

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/mpirun.mpich - from package: mpich

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