postlock: 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/postlock - from package: postfix

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

On Debian possible default full paths are:

/usr/sbin/postlock - from package: postfix

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

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