Ran into this same issue Had to update the runner to macos-13 macos-latest defaults to version 12 which does not support xcode 15

I ran into this same issue, the problem was with github actions. We had our runner set to macos-latest. This uses macos version 12 which does not support xcode version 15. Had to update the runs-on to macos-13 and in our Fastfile we specify the version of code to 15.2.