Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You tried to add an item to a map, but you did not say what to call it. Here is the problem. A list is like a row of boxes: you can drop a new item at the end, and its position is its identity. A map is more like a cabinet of labeled drawers. Every member lives under a key. So when you call data.add on a map with a bare value, rush has no drawer to put it in. It could invent a key, like item one, but then your data would carry a name you never chose, and finding it later would be a guessing game. So rush refuses, and names the collection in the message. The fix has two forms. If you meant a map, give the item a label, with data.add, then the map, then a key, a colon, and the value. If you really wanted an ordered row of things with no names, make the collection a list instead. Say a map of colors that you keep by name. Add red colon the hex code, and the drawer is labeled. Labels are what make a map useful.
Text from the Rush engine's error catalogue.