tracks2prob: 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/tracks2prob - from package: mrtrix

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

Error in other languages:
tracks2prob: bevel nie gevind nie
tracks2prob: comandă negăsită
tracks2prob: comando não encontrado
tracks2prob: commande introuvable
tracks2prob: command not found
tracks2prob: käsku ei ole
tracks2prob: không tìm thấy lệnh
tracks2prob: komanda nerasta
tracks2prob: Kommando nicht gefunden.
tracks2prob: kommandot finns inte
tracks2prob: komut yok
tracks2prob: nie znaleziono polecenia
tracks2prob: níor aimsíodh an t-ordú
tracks2prob: no se encontró la orden
tracks2prob: no s'ha trobat l'ordre
tracks2prob: opdracht niet gevonden
tracks2prob: parancs nem található
tracks2prob: perintah tidak ditemukan
tracks2prob: príkaz nenájdený
tracks2prob: příkaz nenalezen
tracks2prob: команда не найдена
tracks2prob: командата не е открита
tracks2prob: 命令找不到
tracks2prob: コマンドが見つかりません