Still happening in iOS 17.4.1, FB13759599

Tried everything in this thread multiple times, the corrected command just fixes command line builds while building from Xcode still gives "error: cannot execute tool 'metal' due to missing Metal Toolchain"

Via Warren Moore, a solution that worked for me: I’ve found that I’ve had to repeatedly use that xcodebuild incantation (after every reboot) and also mount the resulting .dmg file (which you can do by calling open path/to/downloaded/Package.dmg from the command line after the download finishes). Then Xcode seems to be able to find it.