Technically, you could run now (as far as I know there isn't a rule about joining the race too late).
However, it's more of a "too late" thing. We're 9 days away from election day. If you enter the race right now, you'll be competing with candidates who has a lot more experience than you, and a lot more time to get their campaign out there.
Another reason is because you're new to the stream, and don't know how it works that well, as well as not knowing you pontetial voters well enough.