Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
Your project says it is in a syntax mode, but the mode it names does not exist. Let me explain. In manifest.rush, there is a config place where you choose how strict the syntax is. If you say nothing, you get the default, which is HIGH. But if you write a word, it has to be one of the modes declared in parse.feml. Here, the word you wrote matched none of them. Or you wrote the setting twice, which puts the project in two modes at the same time. In both cases the project is in no mode at all. The engine does not guess which one you meant, and it does not quietly fall back to the default, because leaving the setting out is a choice, while writing a wrong word is a mistake. It refuses before reading a single statement of your code. The message shows the leaf, the word you wrote, and the legal words. For example, a small typo in the mode name lands you here. Pick one of the legal words, write it once, and your project is back in business.
Text from the Rush engine's error catalogue.