where is the needed sdk?

[rmgls]

   Hello all,


Please how can i solve this problem:
   macports 2.5.4
   sudo port selfupdate
   sudo port upgrade outdated
Error: Unable to determine location of a macOS SDK.
Error: Unable to open port: can't read "configure.sdkroot": Unable to determine location of a macOS SDK.
Error: Follow https://guide.macports.org/#project.tickets to report a bug.
   xcodebuild: 
Xcode 10.0
Build version 10A255
   uname -a
Darwin ... 17.7.0 Darwin Kernel Version 17.7.0: Thu Jun 21 22:53:14 PDT 2018; root:xnu-4570.71.2~1/RELEASE_X86_64 x86_64

Thanks

   Best
   Raoul