Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Someone tried to kill the one process that cannot die. Every engine has its own gtr, the first one, born when the engine starts. It holds the materialized laws and the catalogue of refusals that every other process is judged against. If it died, there would be nobody left to judge anything. So kill, vanish and resurrect are refused when they target it. The refusal does not come from a special case written in some corner. It comes from the same single door that every gtr dies through. That door refuses the self slot by its very construction, and it counts the attempt, so you can see how many times someone tried. You can read that count with the rush gtr command, under retires refused. The message names the verb you used and the door's reason, which is either the engine's own, or a slot that holds no living process. If you saw this by accident, check which identifier you passed. And if you wanted to stop work, stop a child, not the engine itself.
Text from the Rush engine's error catalogue.