Well a few thoughts on this. You could set up such a system small scale, probably - for instance in a large space station. The Russian Bios-3 experiments particularly tested a system with just plants grown by humans and they survived reasonably long term and they got most of the nutrients they needed, though they needed some supplements.

And you can grow the plants with hydroponics or aeroponics. But that involved "gardening" and a fair bit of machinery, their aim was to grow as much as possible in a tiny space.

Still that it works would seem to suggest that it's at least possible a system consisting of just humans and plants would work.

And plenty of people are able to survive on a vegeterian diet - though there's a problem with vitamin B12 - you can't get it from yeast, plants, or seaweed, not in a form that humans can use, so would need to get it somehow from bacteria. Still that would count as a "simple form of life" presumably. So I don't see B12 as a problem really.

Maybe you could even manage with just algae and humans if it is a matter of just surviving but being unhealthy. Problem is that algae don't produce all the nutrients humans need. Maybe they could somehow be genetically engineered to do so?

There's a grim short story by Larry Niven called Inconstant Moon: Bordered in Black which describes this idea of an ecosystem consisting of just algae and humans. Though a system like that would also have to have many other symbiotic microbes that live with humans as we can't survive without them. E.g. in our gut so we can digest food. So it's really algae + humans + human microbiome.

Another thing is - how long would it remain as a planet with only humans.

That's another idea explored in a short Sci. Fi. story that I can't remember the author of - that if you had just humans + a planet with only plants in it - then the humans would diversify and a few millions of years later or perhaps say a hundred million years later, you'd find descendants of humans occupying all the niches animals and birds do here - human originated woodpeckers pecking away at trees and human originated mice like creatures in the grass, and human originated herbivores adapted to eat grass etc. Many would have lost their intelligence as it is probably not optimal for survival if you just need to be a herbivore or a woodpecker, to have to provide for a relatively massive brain.

That may seem a bit dystopian, but it's not really that difference from our present situation where all the animals are distant cousins of us. The only difference is that the tree connecting them goes into the future from us instead of sideways via the past. And would be over such huge timescales that there wouldn't be anyone who is human with a child that's a mouse or a woodpecker or whatever.

In the story then the creatures all had human features, but that's unlikely surely. They'd be at least as different from us as primates are and on such long timescales - well probably as different as any large multicellular creatures are on Earth.

(If you know the title or who wrote this story do say in the comments).

The other thing is how the ecoystem works on long timescales. Nitrogen fixation and CO2 fixation can all be done by plants and microbes. Our planet survived for billions of years with just microbes, and green algae produced the oxygen before there were complex animals to breathe it. So an ecosystem of just microbes is possible, and add in humans and they can breathe the air, add in plants as well, I don't see why it can't still work as an ecosystem.

Interested in other answers here, which is partly why I'm answering this question as it's not had any answers yet. Hope this helps a bit or is interesting anyway.