The latest upgrade of LXDE components split the 'lxsession' so 'lxpolkit' is available as a separate package now.
If the Upgrade process will not install 'lxpolkit' automatically, install it manually:
sudo apt install lxpolkit
Without that, you can not run packages with root privileges on LXDE desktop, such as: mounting external devices and partitions, Synaptic, Gufw, root terminal, etc., etc.
Thank you for the heads up! :)