Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You typed a command after rush, and the engine could not run it. Let me explain what is allowed after the word rush. It can be a command declared in the manifest, and rush help lists them all. It can be the path to a script. Or it can be a project directory. If the first word is none of those, the engine refuses, and it does so with a proper catalogued message, never with a bare, unexplained error line. The same goes for a command that is missing an operand it needs. There is one special case. If the word looks like a path, with a slash in it, or ending in dot rush, and nothing readable is there, the engine does not call it an unknown command. It correctly treats it as an unreadable script. The message names what was wrong, and where the right spelling can be found. So try rush help, look for the command you meant, and check the spelling and the operands. Then run it again.
Text from the Rush engine's error catalogue.