rx: 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/rx - from package: lrzsz

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

On Debian possible default full paths are:

/usr/bin/rx - from package: lrzsz

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

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