Nikon D3300 - Manual (Page 273)

253 B The Setup Menu: Camera Setup
If Release locked is selected, the shutter-release button is only
enabled when a memory card is inserted in the camera.
Selecting Enable release allows the shutter to be released when
no memory card is inserted, although no pictures will be
recorded (they will however be displayed in the monitor in
demo mode).
Slot Empty Release Lock
G button
➜ B setup menu