Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your call mixes roles, and some of its arguments are missing their labels. Let me explain the law. If every argument fills a being, the same kind of thing, the call is homogeneous and needs no labels. Concatenating two texts is the example. But if the call also fills a parameter, a different kind of role, then it is heterogeneous, and you must label at least all but one of the arguments. With three arguments, two labels. With a thousand, nine hundred ninety nine. That is the minimum. You can label more, and nobody punishes you. And here is the trick: the label belongs on the parameter, not on the being. So for a power function, write the exponent with its label and leave the base bare. Labeling the base instead is the same mistake, just louder. When you use method form, the receiver fills the being's slot by itself, so a split on a text with just a separator stays bare. The message names the call and what is unlabeled. Add the missing label to the parameter, and the call reads clearly.
Text from the Rush engine's error catalogue.