See wiki:FAQ#prerelease.
Also, observe the part of the log where it says:
:debug:sysinfo macOS 14.0 (darwin/23.0.0) arch i386
:debug:sysinfo MacPorts 2.8.1
:debug:sysinfo Xcode 14.3.1, CLT none
:debug:sysinfo SDK 14
:debug:sysinfo MACOSX_DEPLOYMENT_TARGET: 14.0
:warn:main The macOS 14 SDK does not appear to be installed. Ports may not build correctly.
:warn:main You can install it as part of the Xcode Command Line Tools package by running `xcode-select --install'.
:warn:main If already installed, update with Software Update, or manually: <https://trac.macports.org/wiki/ProblemHotlist#reinstall-clt>
You said you were running Xcode 15.0 beta 8, but MacPorts is detecting that you have Xcode 14.3.1. You'll need Xcode 15 and its version of the command line tools for macOS 14.