debian 10 - Activate tap to click on touchpad?

Debian Stretch and Buster (updated)

Remove the xserver-xorg-input-synaptics package. (important)

# apt remove xserver-xorg-input-synaptics

Install xserver-xorg-input-libinput:

# apt install xserver-xorg-input-libinput

In most cases, make sure you have the xserver-xorg-input-libinput package installed, and not the xserver-xorg-input-synaptics package.

As root:

create /etc/X11/xorg.conf.d/

mkdir /etc/X11/xorg.conf.d

Create the 40-libinput.conf file:

echo 'Section "InputClass"
        Identifier "libinput touchpad catchall"
        MatchIsTouchpad "on"
        MatchDevicePath "/dev/input/event*"
        Driver "libinput"
        Option "Tapping" "on"
EndSection' > /etc/X11/xorg.conf.d/40-libinput.conf

restart your DM; e,g:

# systemctl restart lightdm

or

# systemctl restart gdm3

Debian wiki : Enable tapping on touchpad