You don't need a marketable idea. YC cares about founders and that's it. Everything else can be taught.
But you're absolutely right that that is on the short list of bogus reasons not to apply.
Well then I have the real version of #1 - I have no meaningful skills. I have enough problems that I can't even see myself as a founder, stuff like lack of soft skills from a disability and massive home responsibilities.
>You don't need a marketable idea
Seems like most of the application is describing your idea.
Many of them turn out not to be marketable, though, and that doesn't necessarily count against you. It's not the high-order bit—the high order bit is the founders. Ideas can and do change.