mb-mpi: 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/mb-mpi - from package: mrbayes-mpi

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

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