To answer the question, I quote Ethan Siegel (
https://www.forbes.com/sites/starts...es-of-einsteins-scientific-life/#27dfd8888db4):
"... Einstein rejected the indeterminate, quantum nature of the Universe. This one is still controversial, likely primarily due to Einstein's stubbornness on the subject. In classical physics, like Newtonian gravity, Maxwell's electromagnetism and even General Relativity, the theories really are deterministic.
If you tell me the initial positions and momenta of all the particles in the Universe, I can -- with enough computational power -- tell you how every one of them will evolve, move, and where they will be located at any point in time. But in quantum mechanics, there are not only quantities that can't be known in advance, there is a fundamental indeterminism inherent to the theory. ... But rather than accept these self-evident facts and try and reinterpret how we fundamentally view the quanta making up our Universe, Einstein insisted on viewing them in a deterministic sense, claiming that there must be
hidden variables afoot. It's arguable that the reason physicists still bicker over preferred "interpretations" of quantum mechanics is rooted in Einstein's ill-motivated thinking, rather than simply changing our preconceptions of what a quantum of energy actually is. ..."