There's a lot of arguing out there about a godless universe vs creation science or a God who created the universe. I won't deal with that. It is irrelevant for a physics question.
Krauss' book is popular science, with a lot of the physics ok, but some of it speculative. Maybe to make it more popular, maybe not. The question is this site is whether the physics about the Big Bang is credible or not, from a physics point of view. The answer is that it is speculation, with accepted mainstream physics just having some papers and so on once in a while on the principle of model building and researching possibilities, and otherwise a lot of controversial speculations.
Some treatment of the physics itself below.
His argument is not backed up well by the fact that something so massive can only exist for much less time than before the beginning of inflation before it disappears. Don't know if he argues in his book that the universe total energy is zero so the fluctuation can happen and last longer, he might or not have stated it.
The problem is that's a bit of a flaky argument because that energy is simply Newtonian kinetic plus potential energy, with a loose General Relativistc (GR) similar pseudo-argument that says that
G - T - dark energy = 0,
where G is the Einstein tensor, and T the stress energy tensor. This is the same as the Einstein Field Equations (EFE) with dark energy included, where someone would argue that it means
gravitational energy - particle/field energy - dark energy = 0, so that is a loose argument that total universe energy is zero, in GR.
Other arguments, and I believe used by Krauss, is that you can get a zero energy condition in k=0 flat spacetime, which measurements have indicated we are very close to or possibly exactly so.
There is very little evidence that it really means anything like that, and that then virtual universes can exist for an infinite amount of time. That's a speculative extension of the virtual particle idea in quantum theory, that something with energy E can exist virtually for a time T, where ET roughly equal to h-bar, the Heisenberg uncertainty principle.
Even worse, the uncertainty principle assumes there is a pre-existing spacetime, and that quantum fields exist in their vacuum states, with those fluctuations being zero level fluctuations of those fields. It assumes something pre-exists. And that the zero energy condition is real.
There is some physics theoretical serious research being done about pre-Big Bang physics, for model building and testing. It's things like the possible existence of string theory artifacts like x dimensional membranes x usually greater than 3 or 4), that pre-existed and were collapsing until t=0 where they got so small (Planckian) that it started expanding again. There's other papers with other such ideas, but it all simply says that if you assume something pre-existing one can construct models of how our universe started. There is of course no current evidence of any of that, but it also cannot be ruled out.
None of this is religion, it is all physics, even if speculative. There is no evidence other than these kinds of speculations works, or other speculations that it can start randomly like inflation did in different part of the universe and form the multiverse, are true. They are fun things to read about, some real physics in them, but very speculative extensions of anything that physics considers accepted physics.
Hope this is not controversial, I mean to indicate that where we are and know is not definitive on the physics of the Big Bang itself, that we still do not have an accepted theory of quantum gravity, and that some research beyond the Big Bang is being done, while too much noise is being made of it.