I’m afraid that this is actually the correct error and the framework shouldn’t contain iOS and iOS Simulator code at the same time. Apple tries to force us to use XCFrameworks for this purpose. They started it in Xcode 11 and just tightened up the restrictions.
The only correct way to resolve this is to rebuild the framework as an XCFramework. Which is easy to do:
xcrun xcodebuild -create-xcframework \
-framework /path/to/ios.framework \
-framework /path/to/sim.framework \
-output combined.xcframework
You can start with a combined .framework. Make two copies of the framework, and use lipo to remove the slices from the binary that are associated with a different SDK.
It is based on the original answer from Apple here.
My particular case is that I’m getting this error using Rome, which produces these frameworks (a possible solution is here). Also, a lot of struggling is going on on the Carthage side.