dwi2SH: 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/dwi2SH - from package: mrtrix

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

Error in other languages:
dwi2SH: bevel nie gevind nie
dwi2SH: comandă negăsită
dwi2SH: comando não encontrado
dwi2SH: commande introuvable
dwi2SH: command not found
dwi2SH: käsku ei ole
dwi2SH: không tìm thấy lệnh
dwi2SH: komanda nerasta
dwi2SH: Kommando nicht gefunden.
dwi2SH: kommandot finns inte
dwi2SH: komut yok
dwi2SH: nie znaleziono polecenia
dwi2SH: níor aimsíodh an t-ordú
dwi2SH: no se encontró la orden
dwi2SH: no s'ha trobat l'ordre
dwi2SH: opdracht niet gevonden
dwi2SH: parancs nem található
dwi2SH: perintah tidak ditemukan
dwi2SH: príkaz nenájdený
dwi2SH: příkaz nenalezen
dwi2SH: команда не найдена
dwi2SH: командата не е открита
dwi2SH: 命令找不到
dwi2SH: コマンドが見つかりません