1) The tray must have a receipt on it;
2) The tray must be on the front counter;
3) Make sure your tray is set to "ready", to do so, press "E" on it, a green check mark will appear, at this moment the AI server should come pick up the tray, but if he doesn't move, follow step 4;
4) Pick up the tray and put it right back down on the front counter, the server will come get it;
5) (Only if step 4 doesn't work)Switch trays, get the receipt and all the items, place on a different one, press "E" again, it should work.