I encountered the same problem even I used the Xcode26 Xcode26.0.1 Xcode26.1(https://developer.apple.com/forums/thread/805547)
after two days try I fixed this problem by followed the methods in this article(https://medium.com/@sergey-pekar/how-to-fix-annoying-xcode-26-not-building-swiftui-previews-error-cannot-execute-tool-metal-due-49564e20357c)
I think there is still has a bug for install Metal Toolchain at least for those migrated from Xcode26 Beta