Are there any complex lifeforms which don’t inflict harm upon another?

I hate this question. this question is unable to take an ecological perspective. There are plenty of organisms that cause harm to a subset of a population under circumstance or temporarily, they then inevitably feed an orthogonal population. It does not even make sense to say "oh this X just cause harm".

It is not a realistic understanding of systems. I would suggest reaching out to an ecologist, or if you want someone to relate it to humans, somebody in "social ecology"

In before "Google Bookchin"

First, “harm” is a purely human concept.

That aside, there is no collaboration without competition, there is no peace without conflicts and vice versa.

This is how living systems evolve.

Digging deeper you’ll find that our world is inherently dual.

And people just follow natural tendencies and reflect that fact like any other system.

But sometimes on a much higher scale (just like we produce energy or food on same high scale).

plants compete for sun and secrete toxins to discourage other plants (try growing tomatoes under a black walnut tree)

bacteria, funguses and such also try to poison other species, sometimes even their own conspecifics.

The standard model of population growth is exponential at short time but flattens out because of resource competition -- so I'd say even the simplest life forms do some kind of harm.

Honey bees, which get pollen from flowers and turn it into honey and in exchange they pollinate the flowers.

This is why they were revered in ancient Greece, Persia and India -- as a mental model for how to develop symbiotic relationships and do trade instead of pillaging other communities.

Humans inflict a toll, but we also can do a lot of stewardship. Probably this kind of existential crisis is why so many religions do things like give thanks for food before eating (because food comes from other living things -- we don't eat rocks).

You realize that by every form turning less ordered energy into ordered energy (themselves) and growing larger, they are by nature letting less be around for others. It's a zero sum game in that regard. Yes there are symbiotic relationships when looked at under a limited lens, but the above still holds.

The only way to avoid conflict is to constantly expand and adapt for (or adapt to) new environments, otherwise competition increases.

Mobility also needs to increase to avoid conflicts which lead to physical harm.

Stoners