Why is Flash doesn't support in Auto night detection mode under Camera?

Auto night detection mode is a Camera mode in Samsung Smartphones which will detect low light situations and adjust the camera and device settings to capture great photos without the flash. You just need to hold the device steady while taking pictures in the dark.

 (Auto night detection mode in Camera screen)

Therefore,It is completely normal if flash is disabled when Auto night detection mode is On in your device.

NOTE: Please note that this function does not apply to recording.