Error message
In the message, %s marks a value that the engine fills in when it raises the error.
The story
You asked rush to run a directory, and the engine could not decide where to start. Every directory that rush runs has exactly one entry: the file called main, or main with an extension, or a file named three dots. Exactly one. Not none, not two. If you have none, there is nowhere to begin. If you have two, the engine would have to pick one, and any rule it used to pick would make the ambiguity legal. Imagine a building with no front door, or two doors both marked entrance: either way, a visitor stops and asks. The message names the directory and tells you what it holds, so you can see which case you are in. The fix: if the entry is missing, create a main file. If you have both forms, remove one of them, or rename it. Keep one clear entry, and the engine will know exactly where the story begins. Running a project becomes simple and predictable every single time.
Text from the Rush engine's error catalogue.