urxvt: 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/urxvt - from package: rxvt-unicode

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

On Debian possible default full paths are:

/usr/bin/urxvt - from package: rxvt-unicode-256color
/usr/bin/urxvt - from package: rxvt-unicode-lite
/usr/bin/urxvt - from package: rxvt-unicode

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

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