After a fresh installation of LXDE on Ubuntu 18.04, the tap gesture did not work for the touchpad of my Lenovo Y510P.
Installing Synaptics as helped to solve the problem:
sudo apt install xserver-xorg-input-synaptics
Previously, xserver-xorg-input-libinput was installed, and I did not manage to configure the tap gesture using this driver, properly.
I found this solution on askubuntu.com.