It looks like you’re missing the ydotool.service unit file in the package. Can you check by running dpkg -L ydotool-custom (the name of the package you chose in step 3.4)?
Unit files are how services are defined in most Linux-based desktop systems, similar to services in Windows.
Ah okay, it’s a user unit then. Try the following:
$ systemctl --user enable ydotoold $ systemctl --user start ydotoold