I had the exact same problem a few days ago, running on iOS 14.0.1 and Xcode 12.0. Problem was resolved by updating to the current Xcode version 12.0.1, unfortunately only for 2 days until now. Got the same problem again, even after restarting my Mac.
On of the following finally helped: (not sure which, for me it worked in this order)
Restarting Iphone
Deleting previous install of the app
Connect Iphone to Mac after Xcode and your project is loaded
Just disconnect after first fail and retry
gabriel309
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