It's amazing how complicated just the O2 cycle is. Basically, we don't yet how to do it without a whole planet being involved.

Like, plants do release O2 sometimes, but they also use O2 as fuel when they grow. Growing a plant requires light. On the earth that's easy, just put it in the sun. On Mars there's no atmosphere and no magnetic field, so if you just put a plant on the surface they'll die. So, you need to grow them underground in a mostly earth-like atmosphere at mostly earth-like pressure lit by artificial lights.

So, you plant a lot of plants deep underground lit by bright artificial lights. Then you need to supply the plants with a lot of water. Some of that water will be released into the air, but some of it will be incorporated into the plant's body. There's a whole water cycle that isn't yet fully understood.

What about the soil? On earth worms and other bugs break down leaf litter and other things into usable soil and bees pollinate many of the plants. So, do you ship up a bunch of bugs? You'd have to supply a whole ecosystem of them so they live in balance. You could go with hydroponics instead, but then you'd need a constant supply of nutrients for the plants, and given the amount of plant matter needed for just one human, that would be a huge supply of nutrients.

I'd love to see another honest, scientifically rigorous attempt at a biosphere project. Building a closed ecosystem on Earth is easy-mode compared to doing it anywhere else, but so far all the Biospheres have been failures. IMO until we can easily do it on Earth, we're nowhere near ready to do it in space, on the moon, or on another planet.