It's been a while since I've been building with Xcode, and I found an error that feels quite unfamiliar.

Specifically, the build succeeded, but an error was found when archiving.

The error message is as follows.

Invalid Bundle. The bundle at 'ProductName.app/Frameworks/UnityFramework.framework' contains disallowed file 'Frameworks'.

Upon searching, it appears that when building in Xcode, the 'Always Embed Swift Standard Libraries' setting within UnityFramework must be set to 'No'.

After making this change and archiving, you can see that it proceeds well.
I hope this helps someone.