Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You wrote a line with just a value on it, and nothing else. Maybe a five, a quoted word, a path, or the word null. And the engine said: that is not a statement. Think about what a statement is. It does something. It binds a name, calls a builtin, routes a decision, or gives an answer. A line that only says five does nothing at all. The engine could have evaluated it and thrown it away, but that would hide a real possibility: you forgot something. Maybe you wanted to bind it to a name. Maybe a call is missing, or the line is a leftover from an edit. Silence would let that mistake live in your code. So the refusal happens at parse, before anything runs, and the message shows the value that stands alone. The fix is quick. If the value matters, give it a job: bind it, pass it, or return it. If it is leftover, delete the line. Every line in your script should do something, and now the engine helps you keep it that way.
Text from the Rush engine's error catalogue.