urxvt-ml: 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/urxvt-ml - from package: rxvt-unicode-ml

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