I'm having this issue today. iPhone 7 32 GB, iOS 14.0.1, Xcode 12.0.1, macOS Catalina 10.15.7. Simulators work fine, but when I try to run on my device, Xcode invariably tells me it cannot connect to my device, and to reconnect it. Devices and Simulators tells me to be "certain that Xcode supports development on this device." I am certain, because it worked fine under iOS 14. It wasn't until I updated to iOS 14.0.1 that things started to break.

I seem to have found a solution, but it isn't ideal. Unfortunately, Running Settings > General > Reset All Settings did the trick. I had to go back through all of my settings again, but at least Xcode is happy with my device again. Maybe this was worth it for the sake of an annual rehash of my phone's settings 😅