arj-register: 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/arj-register - from package: arj

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

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