This Error message occur when your iPhone is busy processing something. For example as below.
If you run Flutter doctor you will see message as below.
Error: iPhone X is busy: Fetching debug symbols for iPhone X. Xcode will continue when iPhone X is finished. (code -10)
To confirm I opened xCode -> windows -> Devices and Simulator and it was showing the progress. Let the progress complete and then confirm by running Flutter doctor to make sure there is no error message.