pkcon: 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/pkcon - from package: PackageKit

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

On Debian possible default full paths are:

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

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

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