arj: 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 CentOS possible default full paths are:

/usr/bin/arj - from package: arj

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/arj - from package: arj

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