Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
A value was about to be given a type that the engine does not know how to carry. Here is the principle. In rush, a type is declared by form. Only a few words can be written onto a value as its declared type: path, string, raw string, and the word of a shape member rule. Each of those names a form that the engine can read back later. Now, if the declaration door is handed any other word, say a made-up one, it names no form, and nobody could ever read it back. Rush could store it anyway, but then that value would carry a label no one understands, and the label would travel with the value forever. So it refuses at the door, naming the word in the second slot. Picture trying to stamp a package with the word fragile-ish, in a language the post office does not read. The fix is to use one of the words that exist. If this came from a builtin you did not write, it is an engine defect, so report the message with the word.
Text from the Rush engine's error catalogue.