Here was the response from customer service. It did not solve issue but was worth a try. Maybe it will help you.
Thank you for contacting Canon product support regarding your EOS 80D. I understand the built-in flash of your camera will pop-up, but not fire. I can assist you with this.
Based on the details of the issue it sounds like the flash bulb may be malfunctioning. I recommend performing a complete reset on the camera for further testing. Please follow the instructions below.
Settings Reset
1. While the camera is on turn the mode dial to "P" (Program mode), which allows all menu options to become available, then press the menu button.
2. Move across the top of the menu to the last wrench tab. (Yellow tabs)
3. Go down the menu list to "Clear Settings" and press the Set button.
4. Choose "Clear all Camera Settings", and press the Set button.
5. To complete the reset highlight "OK" and press the Set button again.
6. Next, move over to the Custom Functions menu list.
7. Reset the "Clear all Custom Func." settings as well.
Power Reset
1. Make sure the camera is off, and then remove the battery, memory card, and lens.
2. Close the battery/memory card doors.
3. Turn the power switch to the "ON" position, and then press and hold down the shutter button for 10 seconds.
4. Release the shutter button, turn the power switch to the "OFF" position, reinsert the battery, memory card, and attach the lens.
5. Power the camera on.
If the flash bulb still does not fire after the resets I recommend sending the camera body in for evaluation. Canon cameras come with a 1 year warranty from the date of purchase for manufacturer defects. As long as the camera has not been exposed to impact, pressure, liquid, or sand damage then it is eligible to be covered under warranty. The following instructions will outline how to send the camera in for warranty evaluation.
