Comment (by Roland Haas):

Robyn needed an darwin18.6.0 architecture file, and update package names in port install and port select, as well as updating osx-macports.cfg to account for the new package names.
This change alone would not have helped (the architecture file is still needed). This is a proposed change for the tutorial which I am currently test running on a Mac VM.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2276/use-port-select-gcc-for-osx-macports