Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You gave a rule to data or to filter, and the engine could not enforce it. Here is how rules work. When you write data with rules inside the brackets, each rule is one of two things. Either it is a word from tungsten's shape vocabulary, like homogeneous, bounded, fifo, or sorted, which are the very words that define the collection's shape. Or it is a piece, a small condition that every member must satisfy, like positive. Your rule was neither. So the engine cannot enforce it. And an unenforceable rule is dangerous: you would believe in a protection that does not exist. So the whole declaration is refused. The message shows the rule that failed, and also both of tungsten's explanations: why it is not a shape word, and why it does not parse as a piece. The same refusal applies to filter, except that filter only judges with a piece, so you get only the parsing verdict. A common cause is a misspelling. Check the rule against the vocabulary in data.vocabulary.feml, fix the word, and run again.
Text from the Rush engine's error catalogue.