mrconvert: 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/mrconvert - from package: mrtrix

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

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