The problem in Xcode 12.4 still persists, I understand that the Swift.package, if it is not declared if it supports tvOS or another platform it assumes a default value, but having an iPhone emulator selected, it fails because it is not able to compile on tvOS doesn't make sense when the target doesn't support it.
I'm testing with Xcode 12.5 beta to see if it works.
This solution not work: https://developer.apple.com/forums/thread/652558?answerId=618201022#618201022
In the SupportedPlatform - https://developer.apple.com/documentation/swift_packages/supportedplatform documentation explain:
By default, Xcode assigns a predefined minimum deployment version for each supported platforms unless you configure supported platforms using the platforms API. This predefined deployment version is the oldest deployment target version that the installed SDK supports for a given platform
Topic:
Developer Tools & Services
SubTopic:
Xcode
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