Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You used an old override, engine followed by a word, and the engine told you it no longer exists. Let me explain the history. Engine jit was a way to turn parallelism off. But in this design, parallelism is on by default and cannot be turned off, so a door out was wrong from the start. Engine walker forced the route that is now already the default, so it did nothing at all. Both were removed. But here is the clever part: the engine still recognizes these two words and refuses them loudly. Why? If it just ignored them, the two words would slide into your script's own arguments, and what the interface sees would quietly change. A removal that hides itself is a lie. So you get a clear refusal, naming the removed override and the replacement. If you want to see how the engine divides your script into parallel pieces, set the DEBUG variable to one, and it will show you the divisions. Remove the two words from your command, and everything runs the standard way.
Text from the Rush engine's error catalogue.