average: 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/bin/average - from package: argyll
/usr/lib/mrtrix/bin/average - from package: mrtrix

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

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