How can future be uncertain if EVERYTHING must happen in accordance with the 'cause and effect' principle?

It is as uncertain as we make it...or unmake it. Given enough power to gather information, ultimately, we should be able to be certain of everything. But until then, the future is just as ignorant as we are. Perhaps that is what makes the future unknown as we sense it; we can't see the totality of existence as it pushes itself forward.