Hello, To all the people with the "ARCHS[@]: unbound variable" error after setting the iOS Simulator SDK to arm64 and who can build for a real device (also for Any IOS Device(arm64)) but can't build the Simulator. I solved my problem following the answer from the user called trishcode in this Stack Overflow post: https://stackoverflow.com/questions/63607158/xcode-12-building-for-ios-simulator-but-linking-in-object-file-built-for-ios What you need to do is: Remove what you did earlier, you MUST NOT set the iOS Simulator SDK to arm64, Set the VALIDARCHS variable with an empty value using a User-Defined build setting, you must set this in the project not in the target (in the main project and Pods project if you are using CocoaPods). Clean and build the project with a Simulator selected (I used iPhone 11). The build will fail. After it fails, go to the project and delete the VALIDARCHS variable (in Pods project too). Check that the VALIDARCHS variable is not present in any project or target. Check the main project and the Pods project. Even though you set the VALIDARCHS variable in the projects, check the targets too because when I deleted it from the projects, the variable was still present in the targets. Like I said, remove VALIDARCHS in both the projects and the targets. After all the VALIDARCHS variables have been deleted, build the project again with a Simulator selected and it should build without the error of "ARCHS[@]: unbound variable". Please check the Stack Overflow post for a better explanation of the solution. Good luck.