xprop: 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/xprop - from package: xorg-x11-utils

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

On Debian possible default full paths are:

/usr/bin/xprop - from package: x11-utils

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

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