postkick: 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/postkick - from package: postfix

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

On Debian possible default full paths are:

/usr/sbin/postkick - from package: postfix

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

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