The fact that we can't predict it creates free will imo.
Heisenberg's Uncertainty Principle in quantum mechanics puts it into a natural law.
The reason why we cant predict it has nothing to do with quantum mechanics. The reason is that we arent technically capable to calculate it.
Actually most scientists share the opinion that quantum mechanics play a very, very insignificant role compared to the classical physical processes meaning that the effects are many order too small to have any influencial impact on brain activities or other classical physical processes. At the macroscopic scale of these processes quantum fluctuations just average out. This is due to decoherence.
And even if quantum mechanics would influence brain activities in any significant way, this would still just add a random layer. It could be an argument against natural determinism, yes - but it would just add randomness on top of deterministic values. So nothing that wpuld lead to any form of free will.