gen_WM_mask: command not found

Have You tried accessing the binary with the full path?

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/gen_WM_mask - from package: mrtrix

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