Don't have an idea? Don't have a startup.
Ideally, problems could be solved and ideas implemented without the need to form a company, get funding, or turn a profit. Those are simply (often optional) measures that must be taken to help realize the end goal.
I disagree with that. Some of the better startups we've funded changed their idea completely during YC.
It's the founders that matter, not the idea. So I'd say instead: if you're not the right sort of person to start a startup, don't start a startup. But of course that is a complicated matter to decide because (a) people often don't know if they are the right sort of person and (b) people change.
Sometimes we even tell people when we accept them that we'll accept them if they do something different. Greplin was one of those.
For some people, the issue may not be that they don't have an idea, but they have a few and need to choose the right one to push forward.