mpiexec.lam: 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.lam - from package: lam-runtime

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

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