what is' zero of the potential …

Zero potential energy means the point at which the perfectly rigid body has zero internal energy.

Perfectly rigid bodies usually gain or loose internal energy (internet energy is a term kept vague here) as they enter electromagnetic(if charged) and gravitational(if of mass) fields. This is due to the inherent properties of said fields and their effects on the given quantity of respective material in the body.

So a perfectly rigid body is said to have zero potential energy if the internal energy inside the body is exactly zero.

But wait, electromagnetic and gravitational fields are pretty much universal, i.e they can reach anywhere even if their strength diminishes. The equation has an 1/r² part. So bring this to zero, we need to make rr extremely high, likely infinity.

So the only point in the universe where this is possible is at some infinite distance from said source of field. But since the universe isnt of infinite size, we don’t have a point where the body can have zero potential energy.