August 2016
Future Bucks.
In order to be able to spend Future Bucks you must first craft all the Demos of a given item - like Holo Tulips for example. Once you have crafted 12 Holo Tulips with Robot Parts you can then purchase more with Future Bucks.
And remember: Crafting Demos with Robot Parts also counts toward upgrading your Printer.
If you can't wait that long you can always swap the task for another one using the little X in the corner of the Challenge, but Challenges alternate between event and normal, so you might get something worse.