If you need to add some new item to an order, please select a running order from left and click on Modify Order. We have a perfect mechanism for modifying an order, please do that from there and please don't be confused to do that here, this is only table management section of an order.
What you can do here:
An order may contain many person sitting in multiple tables. a) You can select multiple tables for an order. b) You can not set person more than available sit for in a table. c) You can proceed without selecting table because some people may can gather, take tea and go out. d) As a table can have availability of several chairs and sometime those are sharable, so you can select multiple order in a table