I was having the same issue, and searching for the solutions everywhere, at last i found this GitHub - https://github.com/Carthage/Carthage/issues/3019 thread.
What I did: cloned my project in a new folder(because I had Xcode 11.7 working on another folder)
ran following Carthage update (which failed).
carthage update --platform iOS
3. Then i added (tmp.xcconfig) file in my project root directory, which contains following two lines.
EXCLUDED_ARCHSEFFECTIVE_PLATFORM_SUFFIX_simulatorNATIVE_ARCH_64_BIT_x86_64=arm64 arm64e armv7 armv7s armv6 armv8
EXCLUDED_ARCHS=$(inherited) $(EXCLUDED_ARCHSEFFECTIVE_PLATFORM_SUFFIX_$(EFFECTIVE_PLATFORM_SUFFIX)NATIVE_ARCH_64_BIT_$(NATIVE_ARCH_64_BIT))
4. Then i exported the config file running this export XCODE_XCCONFIG_FILE=$PWD/tmp.xcconfig
5. Finally i ran carthage update --platform iOS --no-use-binaries --cache-builds
Which fixed the issue and i could able to build successfully in Xcode 12.0