https://youtu.be/Xo232kyTsO0 check out this PBS spacetime explanation for what it means for energy to have mass. It's impossible to convert energy into mass, adding up all the mass inside a composite does not equal the true mass, because energy itself has mass. When we split an atom, we are releasing some of that extra (energy) that was stored as potential energy holding the parts together, not converting energy into matter, just moving states of energy to other types.