Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You declared a holder on the filesystem, and rush stopped with: names a leaf, and no leafs mount is declared. Let's unpack that. By default, a filesystem holder in rush rests on leafs. That means the path you wrote is a leaf path, not an ordinary operating system path. But a leaf path needs a place to live, and that place is the leafs mount. Where is it mounted? Only your project knows, and it says so once, in the manifest dot rush file, with a line like leafs equals, and the mount. If there is no manifest above your script, or the manifest never mentions leafs, then rush has nowhere to put your data. It could have dropped the files in the current folder, or into the database, but that would be a lie about where your data lives. So it refuses, by name. You have two fixes, and the message lists both. First, add the leafs line to your manifest so the default has a home. Second, say plainly that you want another filesystem, by giving which and path on the holder. Pick one, run again, and your data lands exactly where you said it would.
Text from the Rush engine's error catalogue.