Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your project calls a function that lives in another project, but nowhere did you say so. Let's see the rule. In project mode, the manifest is the one place that says what a project offers to the world, and it also says what the project depends on, in a requires list. If your code reaches into a peer, for example peer dot lib dot tools dot shout, then that function should appear in requires. If it does not, you have a dependency that nobody wrote down, the kind that surprises the next person, or the next version of you. So rush refuses to load the project, and names the function it found. This applies to projects. A loose script, or the shell, declares nothing, and may reach as it always could. The first slot shows the function and the manifest, and the second slot gives the line to add. Copy that line into your requires list, stating the contract you call it under, and the project loads. Now the dependency is visible, and rush can check it for you every time.
Text from the Rush engine's error catalogue.