Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked rush whether a value follows a rule, but the rule itself was not written properly. Here is how the check works. The builtin is judges a value against a rule and answers true or false. It is useful because your script can collect the values that break the rule, instead of being stopped at the first one by a rule on a bind. Notice who is at fault when something goes wrong. If the value breaks the rule, that is not an error: the answer is simply false. But if the rule is malformed, that is your mistake, and rush refuses and names it. A good rule is a type word followed by its operations, such as number, ge one, le sixty-five thousand five hundred thirty-five, or string dot nonempty. A text that is not a full rule chain, or a shape with a member that is not a rule, is refused. So is a call that asks for both a kind and a rule, since one question per call keeps things clear. Fix the rule, and is will answer.
Text from the Rush engine's error catalogue.