The Apple App Icon guidelines have changed with the release of new iPhone X, iOS 11, and Xcode 9. This error occurs when publishing an IOS app with Xcode 9.
iTunes Store Operation Failed
ITMS—90704: ‘Missing Marketing Icon. iOS Apps must include a 1024x1024px Marketing icon in PNG format. Apps that do flot include the Marketing Icon cannot be submitted for App Review or Beta App Review.’
The workaround is quite simple. In Xcode 9 :