Comment # 1 on bug 973755 from Bj��rn Lie

Open a terminal

xinput list

then do
xinput list-props xx <- == id that is your touchpad

--
As an example, my output

bjolie@drude:~> xinput list
��� Virtual core pointer                        id=2    [master pointer  (3)]
���   ��� Virtual core XTEST pointer                  id=4    [slave  pointer  (2)]
���   ��� ELAN Touchscreen                            id=10    [slave  pointer 
(2)]
���   ��� ETPS/2 Elantech Touchpad                    id=12    [slave  pointer 
(2)]
��� Virtual core keyboard                       id=3    [master keyboard (2)]
    ��� Virtual core XTEST keyboard                 id=5    [slave  keyboard (3)]
    ��� Power Button                                id=6    [slave  keyboard (3)]
    ��� Video Bus                                   id=7    [slave  keyboard (3)]
    ��� Power Button                                id=8    [slave  keyboard (3)]
    ��� WebCam SC-10HDP12B24N                       id=9    [slave  keyboard (3)]
    ��� AT Translated Set 2 keyboard                id=11    [slave  keyboard
(3)]
bjolie@drude:~> xinput list-props 12
Device 'ETPS/2 Elantech Touchpad':
    Device Enabled (136):    1
<snip away lots of output>

    libinput Scroll Methods Available (291):    1, 1, 0
    libinput Scroll Method Enabled (292):    1, 0, 0
    libinput Scroll Method Enabled Default (293):    1, 0, 0

----

If I want to change to edge scrolling I'd do 

xinput set-prop 12 --type=int --format=8 292 0 1 0

(Note that this is for my hardware, for yours it is probably different)

Please paste your entire log, and we can take a look.
man libinput will give you more info.

If you are sure you want to use edge instead of two-fingerscroll you can edit
/etc/X11/xorg.conf.d/90-libinput.conf as root

Section "InputClass"
        Identifier "libinput touchpad catchall"
        MatchIsTouchpad "on"
        MatchDevicePath "/dev/input/event*"
        Driver "libinput"
        Option "Tapping" "On"
EndSection

add

Option "ScrollMethod" "edge"

aka
Section "InputClass"
        Identifier "libinput touchpad catchall"
        MatchIsTouchpad "on"
        MatchDevicePath "/dev/input/event*"
        Driver "libinput"
        Option "Tapping" "On"
        Option "ScrollMethod" "edge"
EndSection

save and reboot.