At first glance, your code is correct. I think this might be the check on our side, that we don't account for absolute paths when preloading the rocket scene.  can you please try to make the preload path relative? Like so:

preload("HomingRocket.tscn")

I'm logging this to look into it and fix the issue for the next release.

Please don´t apologize, and please keep telling us when you face issues. We're here to help, and that also helps us improve practice checks!