Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked to read your mailbox, and rush refused part of the request. Let's see what message.read does. It looks into your own heap, in your own mailbox, to see whether the delivery service dropped a letter there. Because letters can arrive at any moment, you tell rush how to wait: forever, until a timeout, or not at all. If you say nothing, rush reads that as not waiting. That is the whole vocabulary. Anything else, like wait colon soon, or wait colon five without the timeout wrapper, is not a way of waiting rush understands, and it will not guess what you meant. Likewise, if your mailbox field cannot be opened by the read door, rush will not make up a letter to satisfy you. It stops and names the problem. The first slot shows which part was refused, the second what is lawful. Picture checking your mailbox every second in a loop, with a typo in the wait. The fix is to use wait forever, wait timeout with a number of seconds, or wait none. Say how you wait, and rush will do it.
Text from the Rush engine's error catalogue.