Debian Stretch / Arch Linux

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

# apt remove xserver-xorg-input-synaptics
## Arch
[root@arch ~]# pacman -Ss libinput
extra/libinput 1.12.3-1 [installed]
    Input device management and event handling library
extra/xf86-input-libinput 0.28.1-1 (xorg-drivers) [installed]
    Generic input driver for the X.Org server based on libinput

Install xserver-xorg-input-libinput:

## Debian
# 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.

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:

# systemctl restart lightdm


# systemctl restart gdm3

Debian wiki : Enable tapping on touchpad
Source: thanks GAD3R