Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Something outside the engine said no, and this is the general message for it. Rush sometimes reaches beyond itself: to read a file, to look at a directory, to read from standard input, or to run another program. Those things belong to the outside world, and the outside world has its own opinions. When it refuses, the message gives you two parts: what was asked for, and the outside's own reason. That might be the operating system's error text, a failure from a subprocess, or simply that the input ended. Notice what the engine does not do. It does not say the builtin refused, and stop there. And it does not make up an empty answer to pretend things went fine. A fabricated empty result would travel through your script and cause confusion much later, far from the real cause. Instead, you see the true reason, right where it happened. Read the second part of the message carefully, because that is where the real story is. Fix the situation outside, such as the path, the permission, or the input, and try again.
Text from the Rush engine's error catalogue.