I’ve been reading a book about military special forces recently, and the sabotage of the heavy water plant in Norway is described as one of the most important operations during the WWII. How important was it really? As the Germans weren’t actually planning to build a bomb at the time.

Hypothetically: if the Germans had perfect knowledge of nuclear weapons construction and dedicated significant industrial resources to the cause they had the potential to build a nuclear bomb during the war.

Realistically: the raid made absolutely no difference. The German nuclear program was a floundering mess, the nuclear weapons program was at best a gleam in the eye of a few researchers. German researchers were clueless on several key aspects of nuclear weapons design, starting from the very basic level of calculating the critical mass of a bare sphere of fissile material (which they got orders of magnitude wrong). If you were to rate the sophistication of nuclear development of different countries during WWII Germany would rank not first nor second but probably about fifth (behind Japan, the USSR, and the UK).

Additionally, partly because of this floundering and partly because of the fundamental dysfunction of leadership in Germany, the resources allotted to nuclear technology development were meager at best. Germany still went out of its way to set aside huge amounts of resources for cutting edge technology development (the V-1, V-2, Me-262, Z3, etc.) However, the resources allocated to technologies that didn't appear to have near-term practical application to the war effort were very limited, and by all accounts nuclear development appeared to be in that category.

Even the Soviets, with vastly more industrial capacity to spare and a much better knowledge of the practicalities of building nuclear weapons (partially from espionage) put their program on the back burner during the war because they couldn't spare the resources.

The fact that the Manhattan Project succeeded in making a bomb during the war, even with the enormous resources of the US behind it and even with the enormous "dream team" of world renowned super-scientists at the helm is still remarkable. Partly this is because the US pursued the project as a shotgun approach, allowing them to overcome obstacles (of which there were many) by simply relying on other parallel lines of development. However, this required resources that no other nation could conceivably have dedicated to the effort at the time, and even then was only able to produce a handful of working weapons very late in the war and well after Germany was defeated by conventional means.

If the heavy water raids had failed or did not occur it would not have materially changed Germany's ability to produce a nuclear weapon in the 1940s (no such ability existed, even remotely). Any impact on the war would have occurred primarily due to changes in decision making from the Allies based on perhaps an increase in unease about the possibility of a German bomb making effort.