I am now using Xcode 11.6, macOS 10.15.6, and iOS 13.5.1.
First the problem was that I was on Xcode 11.4. But I couldn't upgrade since I wasn't on macOS v10.15 (Catalina) yet. (And I couldn't upgrade because my RAID went down and I couldn't make a backup, but that's another saga.)
After upgrading to Catalina, then to Xcode 11.6 I still couldn't build to the device. So I opened Devices and Simulators and unpaired the phone, as mentioned in the comments here. Then when I tried to re-pair, a warning message said that the device was busy (it was not). Finally, after rebooting the phone (had to untether it for it to come back on), cleaning Xcode, and re-pairing the phone, I finally, finally built to the device. So good luck!