There's a more interesting question hidden there, is there true randomness?

If there's true randomness, then clearly we can't predict the future via the method you described, by taking everything into account and computing according to the laws of nature the next step.

Notice this does not answer whether we have free will or not. It is possible that which is truly random is not related to the human brain. Our thoughts could be the result of predictable chemical reactions. Randomness could affect our thoughts, but the outcome would still be predictable. That would mean brain-simulating AI is possible.

Leaving humans aside, you're talking about absolute knowledge. You assume there's someone with all the necessary knowledge (which amounts to infinite) to predict the future from knowledge of the present, and then you draw conclusions from that. You consider whether such entity exists or not, but you don't consider whether such entity could exist.