pkmon: 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/pkmon - from package: PackageKit

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

On Debian possible default full paths are:

/usr/bin/pkmon - from package: packagekit-tools

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

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