Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You called map or reduce, but the expression you gave never mentions the element. Let's make that concrete. Map and reduce take a small expression and run it once for every member of the collection. For that to make sense, the expression has to talk about the current member. You do that by naming it: x, it, item, or the collection's own label. Inside a reduce, you can also use acc for the running total. Now, if you wrote map with just the number five, or map with some unrelated name, the expression answers the same thing for every member. Five, five, five. It is not a question about the member at all, so the engine refuses instead of quietly repeating the answer. It also means you cannot pass a function here. Only an expression, written right in place. The message shows the argument as you wrote it and the row's own words. The fix: use the element in your expression, for example, x times two. Then map will do its work.
Text from the Rush engine's error catalogue.