It seems that the image is broken in your post.
Could you let us know the error message again? Also, is the Add Note action set to propagate to child?
When you say "The Child Incident is closed" do you mean that the Child Incident is in Status: Closed?
I mean that the Child Incident is open on the screen, but in the Status: Closed.
Apologies for using confusing terminology when I posed the question.
Thank you for clarifying. Could you also please report the image with the error message as this seems to have gone broken in your post.
Also, is the Add Note action set to propagate to child?
Is the Add Note action available at the 'Closed Status'? The action may need to be on the 'Closed' status to propagate to the child.