Impact vfx is not recognized as a game object and cannot be selected using selecta in block prefabs(the serialized field) nor can it be dragged onto it.
Did you unzip the file? And do you use the same version of Unity as Rick or a higher version?
Please do not use an alpha or beta version of Unity for your actual projects. They are full of bugs. Downgrade to a stable version with an “f” in its version number.
The icon of the Impact-VFX looks a bit strange. I don’t know if that’s the new icon in Unity alpha. What file name extension does it have?
It says “.unitypackage”
As for downgrade,I have some data problems so it will take some time for me to download it and tell you if it worked or not.