What does xcode mean "interoperabilityMode is unavailable"?

Trying to write an SPM package using the new C++ interop features using XCode 15 beta when I put in swiftSettings: [.interoperabilityMode(.Cxx)]) I get the error "interoperabilityMode is unavailable" what does that mean? Do I need to enable something in xcode settings to get the new C++ interop stuff?

Needed to change the first line in the package definition to:

// swift-tools-version: 5.9