lxpolkit: command not found

Have You tried accessing the binary with the full path?

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 Debian possible default full paths are:

/usr/bin/lxpolkit - from package: lxsession

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