Black holes do have mass. What they don’t have is volume. The largest known black hole (TON 618) has the mass of more than 60 billion suns. The diameter of the event horizon is so large that you could line up 30 solar systems side by side in it with room to spare. You get the point. Gravity is nothing more than curved space time and black holes curve space time tremendously.

Quantum mechanics is the polar opposite of general relativity. Max Planck, not Einstein, is considered the father of quantum mechanics. Einstein was skeptical of quantum mechanics since it is largely based on probability. One of his more famous quotes regarding quantum mechanics was “God does not play dice with the universe." Hope that helps.