We know how to calculate gravitational forces, but we do not know how to integrate gravity into the mathematics of the quantum theory of the Standard Model. (The fact that we have not seen the graviton yet is not a surprise in the Standard Model, because the graviton has extremely weak interactions, so is rarely produced and rarely detected.)

In the same way that Isaac Newton's laws of mechanics were not wrong but needed to be extended by Einstein to be more accurate about very high velocities, we need to extend the Standard Model with a new theory that will explain gravity thoroughly.