Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your manifest contains a setting that rush has never heard of. Let's talk about why that is an error and not just a shrug. The manifest dot rush file holds your project's configuration through the config call. Each word you write there is a dimension, a setting the engine actually reads, such as syntax mode, or entry. If you write a word that is not on that list, say banana dot split, nothing in the engine will ever look at it. The easy path would be to ignore it. But then you would believe a setting was on, while it was never applied, and a configuration that quietly does nothing is a configuration that lies. So rush refuses before it even parses a single source file. The first slot names the manifest, the line and the unknown word. The second slot lists the real dimensions. Most of the time the cause is a typo or a name from another tool. Compare your word against the list, correct it, and run again. Now your configuration says exactly what it does.
Text from the Rush engine's error catalogue.