tensor2vector: 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/tensor2vector - from package: mrtrix

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

Error in other languages:
tensor2vector: bevel nie gevind nie
tensor2vector: comandă negăsită
tensor2vector: comando não encontrado
tensor2vector: commande introuvable
tensor2vector: command not found
tensor2vector: käsku ei ole
tensor2vector: không tìm thấy lệnh
tensor2vector: komanda nerasta
tensor2vector: Kommando nicht gefunden.
tensor2vector: kommandot finns inte
tensor2vector: komut yok
tensor2vector: nie znaleziono polecenia
tensor2vector: níor aimsíodh an t-ordú
tensor2vector: no se encontró la orden
tensor2vector: no s'ha trobat l'ordre
tensor2vector: opdracht niet gevonden
tensor2vector: parancs nem található
tensor2vector: perintah tidak ditemukan
tensor2vector: príkaz nenájdený
tensor2vector: příkaz nenalezen
tensor2vector: команда не найдена
tensor2vector: командата не е открита
tensor2vector: 命令找不到
tensor2vector: コマンドが見つかりません