Came across the same issue after updating my iOS device to the latest OS, the phone needs to be setup for development again. It may take some time for the setup to complete once plugged back into Xcode, check under Window > Devices.. in Xcode:
As indicated by the alert, to enable Developer Mode go to Settings > Privacy & Security on the iOS device. Scroll down to the Developer Mode list item and navigate into it. To toggle Developer mode, use the “Developer Mode” switch. Tap the switch to enable Developer Mode.
coreyccollins
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