The synaptic Xorg driver in being superseded by the newer libinput driver, you can read more about here: https://wiki.debian.org/SynapticsTouchpad#Change_to_libinput_Xorg_driver_in_Debian_9_.22Stretch.22
You must check the
/etc/X11/xorg.conf.d directory that if there is a file like that XX-libinput.conf.
If the directory doesn’t exists, create the directory first:
sudo mkdir /etc/X11/xorg.conf.d
And copy the sample libinput.conf file with the command:
sudo cp /usr/share/X11/xorg.conf.d/40-libinput.conf /etc/X11/xorg.conf.d
then logout the X session and login again.