I was using a mouse on my notebook and everything was fine. But after removing the mouse, the touchpad wasn't working anymore.
I installed Tweaks using:
sudo apt install gnome-tweaks
to check the status of the touchpad option, but it was already active, so I couldn't do anything.
My notebook is an Acer Aspire 5 A515-54G-53GP, but I don't think that the problem is hardware-related, since the notebook is just over 1 month old.