mrmult: 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/lib/mrtrix/bin/mrmult - from package: mrtrix

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

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