Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You called a function from outside its module, and that function is private. Here is the rule for projects. A function is public only if the manifest dot rush at the project root lists it. Otherwise, the engine treats it as private, everywhere. That is the safe default, and a wise one. The manifest is itself written in rush: a first line with the shebang, a second line with domain data, and then a public list, where each item is the full dotted name of a function. Say a validator function lives in a lib module, and you want others to call it. Add its full name to the list. Private functions work fine from inside their own module, but from anywhere else they are refused at parse, before anything runs. The same applies when you call another project: only what its manifest lists is reachable. Loose scripts and the shell have no manifest, so everything there is public. The message names the function, its module, and the manifest line that would open the door.
Text from the Rush engine's error catalogue.