X: 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/X - from package: xorg-x11-server-Xorg

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

On Debian possible default full paths are:

/usr/bin/X - from package: xserver-xorg

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

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