I managed to get round this by copying
Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/13.4
from the latest Xcode.xib into my installed Xcode package.
Over the last few years Xcode has dropped support for older versions of macOS very aggressively. It requires upgrading to a new major version within the year that the new version came out, and quite often requires the latest minor version as well.
your probably not missing much, there are some serious regressings in that release, for example [0]
[0] https://forums.swift.org/t/swift-5-2-struct-property-wrapper...
(for me, cant make release builds because the compiler crashes... sigh...)