mlock: 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/sbin/mlock - from package: uw-imap-utils

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

On Debian possible default full paths are:

/usr/bin/mlock - from package: mlock

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

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