Why pair production and annihilation cannot occur in free space?

See this discussion threat..

However, quantum mechanics does allow the violation in a very short time to produce virtue pairs that annihilate each other.

As the pressure in space is typically one hundred billionth of that on Earth, pair production is consequently less likely. A good explanation of why such a strong electric field is required lies beyond my understanding of quantum mechanics but you could try here to start.

This is a duplicate question so far as pair production process is concerned.
See answer here. Same logic is applicable for pair annihilation.

Space constantly produces particle-anti particle pairs. They are called virtual particles because they are very short lived and quickly annihilate each other.

Although this appears to violate conservation laws it actually doesn't. The Heisenberg uncertainty principle allow energy to be borrowed for a short period of time. This accounts for the mass of the virtual particles.

An interesting consequence of this is if virtual particles are created near a black hole. If one particle enters the black hole and the other doesn't then the black hole must lose mass to compensate. This is Hawking radiation which can ultimately cause black holes to evaporate.