Greetings, Umidd9595.
It appears that you're unable to use the rear camera on your iPhone because you're receiving a message that "Flash is Disabled" and the phone needs to cool down. I'm happy to help.
Get help with the camera on your iPhone, iPad, or iPod touch
Your flash isn't working
Use Control Center to test the LED flash. Turn the flashlight off and on. If the LED flash doesn't work as a flashlight, contact Apple Support.If the flash isn’t working, or it goes off every time you take a photo, check your flash settings. Tap in the upper left of the screen to choose a different setting.
If you take extended video with the flash on, or you’re using the device in a very warm environment, you might see a message that says the flash is disabled. The message remains until your device cools down.
Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures -- Review the information in this article to learn how to keep your iPhone within an acceptable operating temperature.
Take care!