Imagine a machine that gives you whatever you want that can reasonably be produced. Imagine a world with infinite resources. Economics exists due to scarcity. If there is no scarcity, then everyone can have whatever they want, whenever and wherever they want. Robots will extract the raw resources, build it, and deliver it to you wherever you want them to. There will be no sales. There will be no excess. Just what is needed whenever it is needed.
Yes, that's the Star Trek vision he explains in the beginning. This doesn't answer the question of why the author in the end of his argument suddenly says that "Robots [...] [delivering] it to you wherever you want" would result in job/money problems. Logic would say that with the need for trade there is no need for money and with robots producing everything for you there is no need for you to work.
Well if you don't work you can't get money. So if there are still other economic activities for which you need money, not supplied by the machine then you are out of luck.
