I had the same problem about two months ago. I wrote this note to myself after fixing it:
I think the file I edited was 60-libinput.conf, having to add or change a line to set tapping to true. See the linked web page.
I think the hardest part was being able to edit that file. I didn't save notes on that. You have to edit as root, but I didn't seem set up for that and had to add myself to sudoers file?