Agreed. This is very strange.

I'm successfully adding the .dSYM to the XCFramework, but are the .bcsymbolmap files needed?

What is the real technical reason(s) why non-macOS platforms don't support nested frameworks? Being able to supported nested frameworks in iOS & tvOS would make my life as a framework developer a lot easier.

I've just confirmed that this error does not occur in iOS 14.3 or 15.0 beta simulators; it seems to be isolated to iOS 14.5.

The previous two suggestions (removing an empty runtime environment variable) worked for me. This appears to be a variable that's generated by Xcode, not something that I accidentally added.

Same thing happens to me when attempting to run an iOS app, written 100% in SwiftUI (if that makes a difference), with a target of "My Mac (Designed for iPad)." It worked fine for the first few days, but now I get the same error. I've restarted, deleted Derived Data, cleared the cache, and all the usual jazz. It will build and run without errors on a physical iPhone X. MBP M1 2020, Big Sur 11.1, Xcode 12.4.