Can't submit apps to AppStore: ERROR ITMS-90534: "Invalid Toolchain

Question

ERROR ITMS-90534: \"Invalid Toolchain.

New apps and app updates must be built with the public (GM) versions of Xcode 6 or later, macOS,

Answer 1

I received this error having archived our app from the command line using fastlane gym. When I archived the app manually using Xcode I did NOT receive the error.

I guess there's something wrong with the arguments/config I am passing to fastlane gym. If I work out what this is I will report back.