Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked a data door to work on a name, and that name does not hold a collection. Maybe it holds a number. Maybe it holds a piece of text. Or maybe it holds only a rule you declared with data brackets, and nobody has added the first member yet. In every one of those cases, there is no collection there to read from, update, or remove from. Now here is a subtle point. The engine does not treat this as an empty collection and quietly answer with nothing. Why? Because in rush, empty collections do not exist. A collection is born with its first member. So no collection and an empty collection can never mean the same thing, and the engine will not blur them. The message tells you the name and the door that asked. The fix is to make sure the name really holds a collection before you read from it, usually by adding the first member, or by checking you spelled the right name. Once the collection is there, the door will open.
Text from the Rush engine's error catalogue.