Can I charge a service charge on the terrace only and not inside?

No. There is one service charge for the whole restaurant, set under Settings → The POS in the section called Service charge on tables, anywhere from 0 to 20 percent. It goes on every table's bill as its own line and never on a counter sale. A table already open keeps the old figure until its next round, so change it between services. The Tables, tabs and the kitchen screen tab in the docs has a guide on the service charge.