The process of nuclear fusion in the sun requires

Nuclear fusion in the Sun's core requires both very high temperature and very high pressure to overcome the electrostatic repulsion between hydrogen nuclei. The core temperature is approximately 15 million Kelvin, which provides the nuclei with sufficient kinetic energy to move at high speeds. Simultaneously, the pressure is estimated at 200 billion atmospheres, driven by the immense gravitational weight of the Sun's outer layers. High pressure is essential because it increases the density of the nuclei, forcing them into close proximity and significantly raising the probability of collisions necessary for the proton-proton chain reaction. Without these extreme conditions, the strong nuclear force cannot overcome the repulsive Coulomb force to bind the nuclei together. On Earth, achieving fusion is difficult because these stellar gravitational pressures must be artificially replicated.

Sources

1. [1] https://en.wikipedia.org/wiki/Stellar_core