I have a strange behaviour with the latest version of catalina/xcode/macport : SDK 10.15 is installed, but mac port doesn't find it...
here are some logs:
$ sudo port upgrade outdated
Warning: The macOS 10.15 SDK does not appear to be installed. Ports may not build correctly.
Warning: You can install it as part of the Xcode Command Line Tools package by running `xcode-select --install'.
$ xcode-select --install
xcode-select: error: command line tools are already installed, use "Software Update" to install updates
$ softwareupdate --list
Software Update Tool
Finding available software
No new software available.