Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your message call has an option that rush does not accept. Let's walk through the options. When you call message, there is only one bare word allowed, and it is two-way. Adding it says: this letter needs an answer. If you want to wait for that answer, you must also say how, with wait. You can wait forever, wait with a timeout, or not wait at all, and if you leave wait out, rush reads it as none. The principle is simple: you cannot receive an answer without declaring how you wait for it. Now, what gets refused? Any other bare word. Any other wait value. And, interestingly, a wait of forever or a timeout on a letter that never asked for an answer. Why? Because nobody would ever answer, so you would wait for nothing, and rush will not invent a meaning you did not write. Imagine sending a plain notification with wait forever. Your script would hang. Instead rush stops you at the call. The first slot shows the word that was refused, the second shows what is lawful. Fix the spelling, and send again.
Text from the Rush engine's error catalogue.