Re: How can I modify this loft bed to make it work?
Welcome to the Bunnings Workshop community @billiebillie. It's fantastic to have you join us and many thanks for your question.
The manufacturer seems to be happy with having one support side missing from the bottom there already. You could assume that if you placed a cross member on the side that is missing then you could remove the other side. I would fix a horizontal timber in the position marked red before removing the other timber.
You could reposition the horizontal member either higher or lower as highlighted in red below. To install the ladder in a different position a small piece of vertical timber will need to be bolted into the bed frame and attached to the horizontal safety rails. Once that is done you can simply cut the horizontals safety rails where the ladder will go. It will basically be mirroring what the manufacturer has done on the other side.
Let me also mention a couple of our very knowledgable members @Brad and @ProjetePete to see if they would like to add to the discussion.
Please let me know if you need further assistance or had questions on the process.