As I understand the basic problem, ‘Classical’ general relativity, which is the theory developed by Einstein in 1915, is a theory where gravitational fields are continuous entities in nature. They also represent the geometric properties of 4-dimensional space-time. In quantum mechanics, fields are discontinuous and are defined by ‘quanta.’ So, there is no analog in conventional quantum mechanics for the gravitational field, even though the other three fundamental forces have now been described as ‘quantum fields’ after considerable work in the 1960-1980. Quantum mechanics is incompatible with general relativity because, in quantum field theory, forces act locally through the exchange of well-defined quanta.