Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked a collection for an item, and there was nothing at that address. Let's go through how addressing works. With data read and data remove, you point at a member using at. On a list, at is a position, and positions start at one: at one is the first member. On a keyed collection, at is a key. Now, what counts as nothing? A position past the end of the list, a position below one, a position that is not a whole number, or a key the collection does not hold. In all those cases the engine refuses. Why not just answer with an empty value? Because an empty answer would pretend the collection once held something there. And for remove, silently doing nothing would claim success while nothing was removed. If you gave remove a list of names, the first missing one stops everything, and nothing is removed. The message shows the collection name and the position or key you gave. Check the numbering, remember it starts at one, and check the key spelling.
Text from the Rush engine's error catalogue.