Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You tried to answer a message, but the id you gave does not name any letter. Here is how answering works. When thread B receives a letter from thread A, the letter has an id: sixty-four hexadecimal digits, the same kind of identifier rush uses for every address. To reply, B sends a new message to A, and adds an answers argument holding that exact id. That is how A, when waiting, recognizes the reply: it searches its mailbox for a letter that answers the one it sent. Now, if the text you put after answers is not a real id, say a nickname or a half-copied hex string, then the reply names nothing, and A would never find it. Rush refuses at the call instead of letting you send an answer into the void. The first slot shows what you wrote, and the second says what is lawful. The fix is to read the incoming letter with message.read, copy its id field untouched, and pass that as answers. Same id in, and the conversation connects.
Text from the Rush engine's error catalogue.