I met with similar problem now and found solution for it.
My warning looked like following:
ERROR ITMS-90725: "SDK Version Issue. This app was built with the iOS 12.0 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.
App was built and uploaded with the newest stable XCode (10.1)
To check what SDK indeed you have installed, use terminal with following command:
xcodebuild -sdk -version
In response you will get printed all SDKs. In my case the problem was double SDK 12.1, but one of them was pointing on wrong path (like for iPhoneOS12.0.sdk):
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS12.0.sdk
Solution
Solution in this case is simple.
- Open /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs
There you will find 2 shortcuts pointing to iPhoneOS.sdk.
Remove iPhoneOS12.0.sdk alias.
Well done.
Probably XCode takes wrong SDK based on aliases available in SDKs folder. In my case solution provided above made it working correctly without any warning.