xglock: 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/xglock - from package: xlockmore-gtk

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

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