Hi and welcome to the forum!
It is hard to figure out without knowing more about your setup. (I know you gave a tutorial link, but without knowing the tutorial already, it is time consuming to search for the right screens (also there is the possibility that you made a mistake while following the tutorial:) ))
But I can try to help by making a guess as to a couple of issues it may be:
1- If you are using the 'collision event' action, then switch to the 'trigger event' action when you make the collider a trigger.
2- If that is not it (like if you're already using the correct action), try adding a rigidbody to both game objects involved in the collision (if they both have not got them already).
Good luck with it and I hope this helps!