Germany was not attempting to build a nuclear bomb because Heisenberg had got his sums wrong. He thought you needed 150 tons of enriched Uranium to make a bomb, which was impractical. After the war he started claiming his mistake was deliberate to deny the Nazis a nuclear bomb.
The problem was that the Nazi's had expelled all the Jewish physicists, and a bunch of others had just left. Consequently, Heisenberg didn't have anyone to double-check his calculations or correct the mistake.
It is also doubtful that Germany could have actually produced the nuclear materials needed for a bomb. The heavy water plant in Norway was destroyed, not that you need a single drop of heavy water to make a bomb. So the idea that you could set up a nuclear pile somewhere in occupied Europe and then separation facilities under Allied bombing is frankly fanciful.