I don’t think you only have one chance, that’s not what the success criteria says anyways. I’m not sure if you meant that you failed the project, or only that the last sentence didn’t meet the success criteria.
So I think you still have 3 days to come up with a mini project idea before the project is considered a failure.
My plan before I joined the forum was to do three mini projects and then do grammar learning as a normal project. I had the idea for arithmetic trees, but I didn’t know what to do for the other two mini projects. I wanted the other two mini projects to be progressively harder, but I couldn’t really come up with ideas for that. In a way I did what you are doing now (privately and non-structured as opposed to you), which was to have a mini project to find mini projects to do. Doing exactly three mini projects wasn’t too important though, what mattered was to have a succession of progressively harder projects. So I would say it was a success even if I didn’t do three mini projects. I changed my plan and I like the way it turned out.