Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your script tried to send a message, but the address you gave is not a real address. Here is how messaging works in rush. The first argument to message is a gtrID, the address of a green thread. It is spelled as sixty-four lowercase hexadecimal digits, the same way whoami and process.spawn show addresses. Think of it as a street address for a mailbox. If you pass something that does not have that shape, a short text, a name, a number, or a hex string with capital letters or the wrong length, then there is no house the delivery service could even look for. So rush refuses at the door, before writing a single letter. Notice the difference. If the address has the right shape but nobody lives there, rush does not refuse. The delivery service goes, and answers back that the person is gone, or that nobody answered. That is a delivery result, not a mistake in the address. So when you see this error, check the first slot: it shows what you wrote. Copy the real gtrID from whoami or from the process you started, and send again.
Text from the Rush engine's error catalogue.