I got the same error too with these spec Xcode 12.5 beta 3 Mac os 11.2.3 Xcode, Terminal, and Simulator using Rosetta (I have several libs that does not support arm64 simulator yet) When building, running the apps in the simulator, it works fine, as it run the UI testing Unfortunately, it shows the same error when running the Unit Test Perhaps it is because of this The system prevents you from mixing arm64 code and x86_64 code in the same process Since when we open the Xcode, the arch type is Intel, but the CoreSimulatorService and testmanagerd is running using arch Apple

Yes, apparently Xcode 12.5 are using Clang 10 that require you to cast it to idNSCopying Example: https://github.com/facebook/FBRetainCycleDetector/pull/95/files And if you are using Cocoapods, FBRetainCycleDetector makes that error, you need to specify the commit hash to make sure it using the latest version ruby pod 'FBRetainCycleDetector', { :git = 'https://github.com/facebook/FBRetainCycleDetector.git', :commit = '32c4afc1' }