rtorrent: 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 CentOS possible default full paths are:

/usr/bin/rtorrent - from package: rtorrent

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

On Debian possible default full paths are:

/usr/bin/rtorrent - from package: rtorrent

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

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