To enable the "Service break" option, you need to set the "High End" availability mode:
from "Settings > Booking rules > Availability mode"
Then open the settings page of the service where you need to enable the break and enable the "Service break" option:
Dragging the dark blue slider you can move the position of the break according your needs.
Dragging the starting or the ending handle of the break, you can enlarge or reduce its duration.