select_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/select_tracks - from package: mrtrix

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

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