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

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

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