X11: 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/X11 - from package: x11-common

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

Error in other languages:
X11: bevel nie gevind nie
X11: comandă negăsită
X11: comando não encontrado
X11: commande introuvable
X11: command not found
X11: käsku ei ole
X11: không tìm thấy lệnh
X11: komanda nerasta
X11: Kommando nicht gefunden.
X11: kommandot finns inte
X11: komut yok
X11: nie znaleziono polecenia
X11: níor aimsíodh an t-ordú
X11: no se encontró la orden
X11: no s'ha trobat l'ordre
X11: opdracht niet gevonden
X11: parancs nem található
X11: perintah tidak ditemukan
X11: príkaz nenájdený
X11: příkaz nenalezen
X11: команда не найдена
X11: командата не е открита
X11: 命令找不到
X11: コマンドが見つかりません