App Store Connect
We identified one or more issues with a recent delivery for your app, "xxxx". Please correct the following issues, then upload again.
Invalid Toolchain - Your app was built with an unsupported version of Xcode or SDK. If you plan to submit this build to the App Store, make sure you are using the GM version of Xcode 9 and the SDK for iOS 11, tvOS 11, watchOS 4, or macOS 10.13 or later. If you are using an Xcode beta version to test your app, make sure you are using the latest supported version. For more information about supported beta versions, view the TestFlight Release Notes (https://developer.apple.com/testflight/release-notes/).
Though you are not required to fix the following issues, we wanted to make you aware of them:
SDK Version Issue - This app was built with the iOS 11.2 SDK. Starting March 2019, all iOS apps submitted to the App Store must be built with the iOS 12.1 SDK or later, included in Xcode 10.1 or later.
The App Store Team