I tried to install the community edition on my mac that has Xcode 10 and the development tools (I regularly use them, and my MacPorts software uses them too).
That is an error with the installation message, it should say supported up through Xcode 9.x; we don’t support Xcode 10 or Mojave at the moment. But you should be OK with 9.x and High Sierra.
Xcode 9 would be your best bet to install 18.10 CE. You can grab it from Apple’s download archives. Xcode 10 and 9 can be installed side-by-side and the “active” one can be picked with xcode-select, e.g. sudo xcode-select -s /Path/To/Xcode9.app/Contents/Developer
This thread is just the detail I needed and I just wanted to close it out regarding the command line - use xcode-select on the command line to choose the version of Xcode you want to work with and I believe you do need the command line tools for each…
$ xcode-select --help
Options:
-h, --help
-p, --print-path
-s , --switch set the path for the active
–install open a dialog for install cmd line tools