xkill: 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/xkill - from package: xorg-x11-server-utils

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

On Debian possible default full paths are:

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

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

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