I have also been having this issue with Xcode 15.4 (release candidate, 15F31c). I'm coding for the VisionPro, and the RealityKitContent (where I put all the bundled usdz files) was marked as missing package. Specifically that the Package.swift was innaccessible as it was not present in the file system. However, I could clearly see it & read it in Finder & Xcode's file browser part. I tried moving Xcode to Applications, which may have fixed it briefly, but the next day the same problem was happening. I tried re-adding the Package manually, but that didn't help, it even complained about adding it because it was not present (same problem as above). However, I eventually worked around it by deleting the git repository completely & re-cloning from my remote copy. I am now back in & it compiled. I just really hope it doesn't do it frequently, as that would be a major pain.