Unfortunately after a denial it is much harder to apply again as they'll check your previous application and look for alterations (i.e. lies or additions).

A business accelerator doesn't really fall into any one visa category well. Is it a 2-3 month "business meeting" (i.e. B-1), is it educational (i.e. F-1), or even a "short course" eligible under a travel visa (B-2)? All of the above?

The reason you got denied is as it says, and while you can re-apply providing more evidence it will be an uphill battle. You just have to show WHY you'll be returning home (e.g. family, a job, other obligations)? By any chance did you list yourself as self-employed, rather than an employee of your own company? They hate self-employed people and often treat them as "unemployed."