Let's restate it. Ordinarily, nothing can exist prior to itself. But there must have been a point in time/space (or "outside" of time/space) in which the initial condition of everything that follows did exist prior to itself.
This is fallacious. There
is no 'prior' to the Big Bag, as spacetime came into existence with TBB. Therefore, the laws of causation and the very premise that 'nothing can exist 'before'), while it might be valid now, would simply not be applicable. This is similar to the fallacy of applying the laws of physics as they exist today to the early stages of the Big Bang- it simply isn't applicable.
That ^ does not rule out ALL (or ANY) "causation" from that initial point onward.
It rules our a cause for the big bang, as causation implies 'since-because' relationship between two incidents at different points
in time (within spacetime, which came into existence with TBB). Such a causal relationship is therefore impossible- including the sense in which you use it. Therefore, causation and by extension creation is impossible. Evena deity could not act as a cause, for the very concept of a cause is invalid without time.