Hi @aevilida,
The log you have attached just indicates that there was no valid BP Fire set on the Barrel, which lines up with you not being able to set it in the first screenshot.
You won’t be able to set a reference to something in the CDO that doesn’t already exist within the CDO. For example, you can reference a Component inside an Actor, but you won’t be able to reference another Actor in the CDO. The CDO being the version of the object you are actually editing in the blueprint editor.
I’m not familiar with the content of that book, but I’m guessing they are trying to get you to set the BP Fire value to something on an instance of a Barrel, i.e. a Barrel that has already been placed in the world, or instantiated.
It’s normal for the value to not be editable in the CDO, just ensure that Instance Editable is enabled.
Then when you place a Barrel in the world, you will get the option to set your BP Fire variable.
I hope this was helpful!
Thanks,
Hayden