Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You pointed at a process, and there is no process at that address. Every process in rush has an address, its gtr identifier, sixty four hexadecimal characters, the whole number, handed to you by process new or by whoami. If you give the engine a text of some other shape, it is not an address at all. And even if the shape is perfect, but no process, living or retired, carries that identifier on this engine, it points at nothing. Both cases are refused by name. Notice one thing the engine does not refuse: acting on a process that has already retired. That is quietly accepted, because a slip should not kill the script. If you tell a finished process to stop, nothing breaks. The message shows the text you wrote and the reason it names nothing. Typical causes: a copy that lost a few characters, or an identifier from a different engine. Copy the identifier again, exactly as received, and the address will resolve.
Text from the Rush engine's error catalogue.