rxvt-xterm: 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 Debian possible default full paths are:

/usr/bin/rxvt-xterm - from package: rxvt

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

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