What's wrong with the first planet in chapter 5?

When Meg arrives on the first planet, she feels an enormous sense of pressure, as if she is being flattened by a steamroller. She can't breathe, because she can't get her lungs to rise and fall. To her, this feels very different from not having enough oxygen. Her mind also feels flattened out, making it difficult to think, and she finds her heart can't beat properly.
All of this occurs because the first planet is two-dimensional, not three-dimensional.
Mrs. Which has taken them there by mistake. Not corporeal herself, though she takes on a human guise, she forgets that the children can't survive on a two-dimensional planet. She simply remembers it as a pleasant place that she, Mrs. Whatsit, and Mrs. Who enjoy visiting.