cat_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/cat_tracks - from package: mrtrix

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

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