We're going to need two sliders for this question.
The first slider defines 'creature'. On one end, viruses, and on the other end, we'll use humans (as an anthropocentric example). You could set the slider to be placed s.t. plants are not considered creatures, or you could set it elsewhere.
The second slider defines what constitutes harming a creature. This can range from purposefully destroying a creature to accidentally harming a creature as an externality of an act of society.
The blunt answer to this question will be that it is impossible if plants are considered creatures, as humans cannot subsist off of anything less complex than plants. In addition, something as 'simple' as mowing a lawn or cutting down a tree absolutely destroys the habitat of hundreds if not thousands of insects and animals, and many will die in the process. Most likely, the intent is not to harm the lives of those depending on the habitat, but as an externality, that is the result.
Let's limit ourselves to creatures being defined as animals and harm being considered direct harm (i.e. accidentally stepping on an ant does not count as 'harm' since the human is likely unaware of their action, and plowing a field does not count since the intention of the action is not to destroy habitats or kill the inhabitants).
First, it is likely that raising animals is not an option. By raising a carnivore such as a dog, you are forcing harm upon other animals in order to feed your dog. By raising an herbivore such as a sheep, you are forced to perform reproductive control or face Malthusian population crashes/mass death due to lack of food, both of which could be considered harmful due to the human either interfering with the animal or watching them all starve.
Second, the above logic applies to humans as well. There are only so many resources and there is only so much space. In your question you noted that there are clean natural environments and plenty of rich fertile fields. The only practical way to support a growing human population is to destroy the 'clean' and 'natural' aspect if the 'space' issue is to be addressed due to the constraints of the problem. After all, if there is not enough food and water, people will have to address the shortage with either technological innovation, culling, or starvation, and only the first option doesn't involve inflicting harm upon other humans.
In short, one possible way for humans to survive and allow their population to expand without harming any animal creature is to use technological innovation to allow for a vegetarian diet with productivity tied to the rate of population growth. In addition, humans will have to refrain from excessive reproduction in order to keep their population growth sustainable.
This is but one possible answer. There are definitely other points that could result in alternate conclusions - the important thing is that whatever solution you choose for your world is internally consistent.