Back camera and flash light can not work

Sorry about that. See also the instructions provided in this support guide:--> If the camera or flash on your iPhone, iPad, or iPod touch isn't working - Apple Support

If you still get no help (after eventually contacting Apple Support directly as per the above guide), have your iphone inspected at an Apple Store (when they re-open after this tragic Covid-19 period).

Regards

Giulio

Have you already updated your iPhone to iOS 13.4?

Among the descriptions that you can find here About iOS 13 Updates - Apple Support , that issue should be resolved.

Regards

Giulio

You are welcome