So if I fill a big, weak tank with a poisonous substance and deliberately park it upstream from a public reservoir, but don't actually open a valve to dump the toxin into the reservoir, and there's really only about a 20% chance of the thing bursting and dumping the whole load of toxin into the reservoir, you are saying that I have done nothing wrong, and should not be subject to prosecution, because although I set up a situation with a real probability of poisoning the water supply, I didn't actually poison the water supply.
ISTM that you are really saying that it's only attempted murder if the toxin actually winds up in the water supply; if so, you don't understand what attempted murder is. It's when you try to kill someone, but fail. I suspect that there's no mens rea here, so the charge wouldn't stick, but knowingly manufacturing a pandemic virus ought to be a crime, and the knowingly part would then constitute mens rea.