Apple platforms use Mach-O as their object file format, so you should think of the “apple mach-o linker” as just the linker. In short, you have a linker error.
You can learn more about linker errors by looking at the error message output by the linker. Xcode doesn’t do a good job of surfacing this through Issue navigator. You can, however, see the actual error in the build transcript, which you see in the Report navigator (View > Navigators > Show Report Navigator). Select By Time at the top, select the most recent Build entry in the list on the left, select the failing Link step in the main editor, then choose Editor > Expand Selected Transcripts.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"