Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You tried to append to something that is not a list. Append works in two small steps: it creates one value, then it attaches a reference to that value at the end of a list. So there must be a list with an end. If the name holds nothing, there is nothing to append to. And note that append does not create the collection for you. The door that births a collection is data add, on its first pour. If the name holds a map, that is also wrong, because a map's members are labeled roles, there is no such thing as the end of a map. For a map, you use data add with a label. Think of a queue at a shop: you can join the back of a line, but you cannot join the back of a row of named lockers. The message names what you wrote and what it actually holds. Check which kind of collection you really have, and use the matching door. Lists get append. Maps get labels.
Text from the Rush engine's error catalogue.