Faced the same issue. The problem was that I was running my macOS system in English (UK), but my Siri language was set to German, so the Apple Intelligence system wasn't activated because there are currently no AI models for Siri in German. Once I switched Siri to English (UK), the macOS AI system could be downloaded and installed. Then the AI module in Xcode started to work. That also fixed my issue, although I had Siri set to English US, had the same error, but after switching Siri to English UK the problem was gone