There’s an issue where Xcode 15.4 fails to build Mac Catalyst apps with certain package dependencies with:

clang:1:1 invalid version number in '-target x86_64-apple-ios10.15-macabi'

Or:

clang:1:1: invalid version number in '-target arm64-apple-ios10.15-macabi'

Workaround: use Xcode 15.3.