mpath: 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/mpath - from package: libmodule-path-perl

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