Unlike the Focusrite Saffire range of FireWire Audio Interfaces, the Scarlett 2i2, 2i4, 8i6 and 18i6 cannot currently run in Standalone Mode.
The 8i6 and 18i6 specifically may pass audio from their analogue inputs to their analogue outputs, through means of Direct Monitoring if MixControl is set up to do so (as it is by default), however this functionality is unsupported and not fully tested. This is not possible with the 2i2 and 2i4 though.
The Scarlett 18i20 will support standalone mode after a firmware update in the future.