It looks like a part is missing.
You really need to take cabinet out and repair. It does in from the top.
Yes, it does look like a section of upright is missing. Where are the other pieces? Laying them all out may answer the question.
I would take out the metal rod and put in shelves. You can find round ones at Lowes or Home Depot.
Why did you take it apart
If that plastic piece sitting on the bottom goes in that pipe, there should be a way to put a spring under it so it hold between the pipe and the stem.