There's still scarcity of authentic human goods and services, even in a world where AI can provide any physical good or service. For example, if you wanted to buy a new statue or art piece a famous artist made or spend time with them, you could pay them money for it. This is a way of keeping tabs on how much people in the optional society of goods owe each other. This economy still has scarcity (there is only ONE authentic item or experience), and many people value that authenticity over a copy. It's the same reason why a copy of the Mona Lisa is worth much less than the real Mona Lisa.