hi.
If both prefabs are in the scene and then you drag in to reference it will work.
But if prefabs are created @ runtime this will not work.
Object in project folder can not communicate directly to object in the scene/hierarchy
So when Prefab is spawned it will need to get a reference to that object 1st.
There are a few ways that you can do this:
Use the "Find game object".
But if you need to do this many times, its bad for performance.
Or
When creating a prefab, store the created prefab in a variable, the use 'Set Fsm Game Object' to set the game object that you wish to reference on that prefab.
There are some other ways as well but the 2nd option might be the best to use.
This is a unity limitation by the way