Build for iOS fails in Xcode 11

Question

I change my iOS SDK to 14.0 from 13.4 then I try to build for ios but it fails when I try to build using 13.4. It doesn't show an error but I cannot upload the app it says error on the picture so needs to upgrade my SDK.

Here is my flutter --verbose:

    [✓] Flutter (Channel stable, 2.0.5, on Mac OS X 10.15.3 19D76 darwin-x64, locale en-US)
    • Flutter version 2.0.5 at /Users/fee/Documents/Developer/flutter
    • Framework revision adc687823a (2 weeks ago), 2021-04-16 09:40:20 -0700
    • Engine revision b09f014e96
    • Dart version 2.12.3

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at /Users/fee/Library/Android/sdk
    • Platform android-30, build-tools 28.0.3
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)
    • All Android licenses accepted.

[!] Xcode - develop for iOS and macOS
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.4.1, Build version 11E503a
    ! Xcode 11.4.1 out of date (12.0.1 is recommended).
      Download the latest version or update via the Mac App Store.
    • CocoaPods version 1.10.1

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 3.5)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 44.0.1
    • Dart plugin version 191.8593
    • Java version OpenJDK Runtime Environment (build 1.8.0_202-release-1483-b49-5587405)

[✓] Connected device (2 available)
    • Android SDK built for x86 (mobile) • emulator-5554 • android-x86    • Android 10 (API 29) (emulator)
    • Chrome (web)                       • chrome        • web-javascript • Google Chrome 90.0.4430.93

And, here is the error shown

Error output from Xcode build:

2021-04-30 13:05:34.432 XCBBuildService[28375:199855] /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.0.sdk: error: SDK 'iphoneos14.0'
    already registered from /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk
    2021-04-30 13:05:34.610 XCBBuildService[28375:199855] /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.0.sdk: error: SDK 'iphoneos14.0'
    already registered from /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk
    2021-04-30 13:05:34.840 XCBBuildService[28375:199855] /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.0.sdk: error: SDK 'iphoneos14.0'
    already registered from /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk
    ** BUILD FAILED ** 

Xcode's output:
↳
    note: Using new build system
    note: Building targets in parallel
    error: unknown error while handling message: unableToInitializeCore(errors: ["/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS14.0.sdk:
    error: SDK \'iphoneos14.0\' already registered from /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk"])

I want your help to find where is this error comes from.

Answer 1

Your flutter doctor basically mentioned the problem for you:

Xcode 11.4.1 out of date (12.0.1 is recommended).

while Xcode 12 is required to build and upload to App Store. Update Xcode and try building again.