Hello Matt,
What directory are you in, when you run the "gcloud app deploy" command? You are expected to be in the top-level directory, where the app.yaml configuration file is present.
You seem to lack a directory, by reference to:
TopLevelDirectory
|- Package.swift
|- Sources
|- MyAppName
|- MyAppNameSource1.swift
|- MyAppNameSource2.swift
A directory named analogous to "MyAppName" seems to be missing.
It may prove helpful, for the sake of investigation, to actually use the "path" property, as advised in the error message, and determine this way which path is missing.