pkexec: 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/pkexec - from package: polkit

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

On Debian possible default full paths are:

/usr/bin/pkexec - from package: policykit-1

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

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