sample_tracks: 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/sample_tracks - from package: mrtrix

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

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