Man, I spent like 30 minutes in that last challenge, with no debug message showing. I realized pretty quickly that I chose OnTriggerEnter() instead of Enter2D() in the IDE but it took me a long time to realize my input was (Collider other) instead of (Collider2D other).
Oh, this happens. The other one is the triggers having a Collider argument, and collisions having Collision. Had this happen a few times before. My IDE does most of the work so it doesn’t happen that often, but changing from collision to trigger (or vice versa) has had me stumped a few times