xmlock: 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/xmlock - from package: xlockmore-motif

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

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