I do not see support for iOS 13.6 in the 11.7 release of Xcode. Just 13.5 and 13.7. I downloaded the zip from the Developer downloads area - no indication that it's a beta - and explored the package, navigating to
Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport .
There is no 13.6 folder.
Solved in my case by updating my iOS development device to 13.7.