threshold: 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/threshold - from package: mrtrix

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

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