Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
The engine read your script, hit something it does not understand, and refused to run anything at all. Let me tell you why it does that. It could have skipped the line it did not recognize and carried on. But then the rest of your script would run, report success, and the output would belong to a different program, one with a hole in it. Nobody would know. That is far more dangerous than a refusal, because a refusal at least tells you the truth. So the rule is firm: if one construct is unknown, the whole script is refused, and nothing executes. The message gives you the file, and also the construct and the line, so you can find it quickly. Say you wrote a statement shape the parser has never been taught: it will point right at that line. What do you do? Check the spelling and the syntax first. If the construct is truly meant to exist, then the fix is on the engine side: declare the node kind in parse.feml and teach the parser. Never loosen the refusal. A loud no is protecting you.
Text from the Rush engine's error catalogue.