Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You gave a builtin a key that it does not have. Every builtin declares its own keys in builtins.feml, and you cannot rename them. In the example, someone wrote write with a key called value. But write has no parameter called value. Its main thing, its being, is text. So the engine refuses at parse and teaches in the message. It lists the real keys of the row. And if your argument would be the row's being, it shows the bare spelling, which needs no key at all: just write, then x. Why is there no key for the main being? Because the rush creators already know what it is, so asking you to write it would be noise. The old message would complain that the name value locates nothing, which sent people looking in the wrong place. Now you get the truth. Check the list of keys in the message, pick the right one, or drop the key altogether when the argument is the being. Clear and short.
Text from the Rush engine's error catalogue.