urxvt256c: 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/urxvt256c - from package: rxvt-unicode-256color

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

Error in other languages:
urxvt256c: bevel nie gevind nie
urxvt256c: comandă negăsită
urxvt256c: comando não encontrado
urxvt256c: commande introuvable
urxvt256c: command not found
urxvt256c: käsku ei ole
urxvt256c: không tìm thấy lệnh
urxvt256c: komanda nerasta
urxvt256c: Kommando nicht gefunden.
urxvt256c: kommandot finns inte
urxvt256c: komut yok
urxvt256c: nie znaleziono polecenia
urxvt256c: níor aimsíodh an t-ordú
urxvt256c: no se encontró la orden
urxvt256c: no s'ha trobat l'ordre
urxvt256c: opdracht niet gevonden
urxvt256c: parancs nem található
urxvt256c: perintah tidak ditemukan
urxvt256c: príkaz nenájdený
urxvt256c: příkaz nenalezen
urxvt256c: команда не найдена
urxvt256c: командата не е открита
urxvt256c: 命令找不到
urxvt256c: コマンドが見つかりません