Learn about flash of camera cannot be open.
1. The flash light will be disabled if battery is low, try to charge your battery to a higher level and try again.
2. Check whether the camera is set to the portrait mode. In order to achieve a certain effect, the program will prohibit use of the flash. Please revert back to other camera shooting mode.
If the issue is not solved, please back up all the data, clear the cache and flash to the latest ColorOS version.