If the flash cannot be turned on in instances when you want to use a fill flash, then the camera is set to an Automatic (Auto) or Intelligent Auto Adjustment mode. When using an auto mode, the camera automatically selects the settings it determines are necessary to record optimal images. In some cases pictures may not be shot properly if the flash is forced to strobe; therefore, the camera disables the flash. In cases where you want to use the flash, turn off the auto mode.
NOTE: If model-specific information is required for how to turn off the auto mode of your camera, refer to the operating instructions supplied with the product. Manuals are posted on your model support page.