Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You passed a collection straight into a call, with no name on it, and the engine wants a name. In rush, a collection should have a holder name, so that anyone reading the script can see what it is. There is one comfortable exception: a collection of plain numbers, all the same kind, may be written right in the argument. So the sum of one, two, three, in brackets, is fine, because that is a calculation. Anything else, a list of texts, a mix of kinds, a collection with labeled members, must be bound to a name first. There is also a second exception: an inline labeled map is fine inside a keyed argument, because the key names it. So a budget key can carry a small map of its own. The message shows what kind of collection you passed. The fix is easy: give it a name on the line above, then pass the name. Say a list of file names: bind it to files, then call with files. Clearer to read, clearer to debug.
Text from the Rush engine's error catalogue.