Maybe the flashlight is disabled in camera's settings. In this case, you need to go to camera's settings and enable it (usually by tapping on the thunderbolt icon).
Note: On most devices, the thunderbolt (i.e. the flashlight settings) is shown on the main camera screen.
Maybe some filters you enabled prevent you from using the flashlight. Try disabling them in the camera's settings and then try using the flashlight.
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